This document was prepared by DLZ Michigan, Inc. under the direction of MDOT s Roundabout Committee which consists of the following individuals:

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2 FOREWORD With the study, design, and construction of modern roundabouts (referred to in this document as roundabouts ) becoming commonplace throughout the state of Michigan, the Michigan Department of Transportation (MDOT) has developed this Roundabout Guidance Document with the intended purpose of assisting roundabout designers and encouraging uniformity in certain elements of design (as appropriate) within the state of Michigan. This document is intended to be used by MDOT staff as they study, design, and build modern roundabouts within the state. It also helps guide the decision process regarding when a roundabout should be constructed or not. This document is intended to serve as a supplement to the Federal Highway Administration (FHWA) document Roundabouts: An Informational Guide (publication number FHWA-RD ) which can be viewed and downloaded at the following web address: Throughout this document, the FHWA Guide is cited and supplemental information is provided where relevant. This document is not intended to serve as a comprehensive and rigid set of design standards. Rather, it is intended to provide general guidance and identify considerations related to some important roundabout design issues. Furthermore, users of this document need to recognize that good roundabout design does not entail a strict pre-defined process with repeated application of the same rules at each intersection considered. Instead, it requires the judicious application of roundabout design principles that help to reach the optimal geometric design at each individual location. This process very often involves tradeoffs between competing objectives to reach the best solution. Good roundabout design requires the use of professional engineering judgment and a great deal of thought on the part of the designer. The use of sound engineering principles and common sense are also vital to successful roundabout design. The use of this document does not in any way relieve the designer of his/her personal responsibility to produce a roundabout design that functions safely and efficiently within the context of a given location. While this document provides direction to designers and identifies the main principles involved in roundabout design, there is no substitute for detailed review by someone with roundabout experience, especially with multilane roundabouts. Users of this document are encouraged to have their roundabout designs (and especially multilane roundabouts) carefully reviewed by an experienced roundabout designer with knowledge in all aspects of roundabouts. Document Preparers This document was prepared by DLZ Michigan, Inc. under the direction of MDOT s Roundabout Committee which consists of the following individuals: Mark Bott Drew Buckner Imad Gedaoun Terry Palmer In addition, a special thanks goes to R. Barry Crown of Rodel Software located in Stoke-on-Trent in the United Kingdom for advice and information in several areas of the document. i

3 TABLE OF CONTENTS SECTION 1: INTRODUCTION... 1 SECTION 1.1: CONTENT AND APPLICABILITY OF GUIDE...1 SECTION 1.2: BASIC TERMINOLOGY...1 SECTION 2: PLANNING... 4 SECTION 2.1: INTRODUCTION...4 SECTION 2.2: GENERAL CONSIDERATIONS...4 SECTION 2.3: BENEFICIAL LOCATIONS AND APPLICATIONS...7 SECTION 2.4: LOCATIONS AND APPLICATIONS WHERE CARE SHOULD BE EXERCISED...8 SECTION 3: SAFETY... 9 SECTION 3.1: INTRODUCTION...9 SECTION 3.2: GEOMETRIC DESIGN AND SAFETY...10 SECTION 3.3: ROUNDABOUT USE AS A SAFETY COUNTERMEASURE...10 SECTION 4: GEOMETRIC DESIGN SECTION 4.1: INTRODUCTION...12 SECTION 4.2: GEOMETRIC ELEMENTS...12 SECTION 4.3: CAPACITY ANALYSIS...12 SECTION 4.4: GENERAL DESIGN GUIDANCE...16 SECTION 4.5: THREE-LANE ROUNDABOUTS...20 SECTION 4.6: ACCOMMODATING TRUCKS...21 SECTION 4.7: CROSS SLOPE...23 SECTION 4.8: SIGHT DISTANCE...23 SECTION 4.9: PEDESTRIANS...24 SECTION 4.10: BICYCLES...25 SECTION 5: PAVEMENT MARKINGS AND SIGNING SECTION 5.1: INTRODUCTION...26 SECTION 5.2: PAVEMENT MARKINGS...26 SECTION 5.3: SIGNING...30 SECTION 6: OTHER DESIGN AND OPERATIONAL CONSIDERATIONS SECTION 6.1: INTRODUCTION...34 SECTION 6.2: LIGHTING...34 SECTION 6.3: LANDSCAPING...34 SECTION 6.4: MAINTENANCE...35 SECTION 6.5: ONGOING MONITORING...35 SECTION 6.6: PAVEMENT TYPE...35 SECTION 6.7: UTILITIES...36 SECTION 6.8: MAINTENANCE OF TRAFFIC...36 SECTION 7: PUBLIC INVOLVEMENT SECTION 7.1: INTRODUCTION...37 SECTION 7.2: EDUCATING THE PUBLIC...37 SECTION 7.3: PUBLIC INVOLVEMENT TECHNIQUES...38 SECTION 8: REFERENCES ii

4 TABLE OF CONTENTS LIST OF TABLES Table 1: Geometric Delay For Roundabouts...6 Table 2: Typical Roundabout Capacities...7 Table 3: Approximate R 4 Value and Maximum R 1 Value...16 LIST OF FIGURES FIGURE 1: COMMON ROUNDABOUT TERMINOLOGY...2 FIGURE 2: GEOMETRIC PARAMETERS...3 FIGURE 3: CONFLICT POINTS AT A STANDARD INTERSECTION AND A ROUNDABOUT....9 FIGURE 4: VEHICLE PATH RADII (SOURCE: FHWA ROUNDABOUT GUIDE)...17 FIGURE 5: TYPICAL THREE-LANE ROUNDABOUT (STERLING HEIGHTS, MI)...21 FIGURE 6: TYPICAL SINGLE-LANE ROUNDABOUT (GAYLORD, MI)...22 FIGURE 7: ROUNDABOUT PAVEMENT MARKING TERMINOLOGY...27 FIGURE 8: ROUNDABOUT PAVEMENT MARKING TERMINOLOGY (CONTINUED)...27 FIGURE 9: TYPICAL TWO-LANE ROUNDABOUT (SOUTH BEND, IN)...29 FIGURE 10: TYPICAL ROUNDABOUT SIGNING (SOURCE: FHWA ROUNDABOUT GUIDE).30 FIGURE 11: ILLUMINATED BOLLARDS...31 FIGURE 12: COMBINED DESTINATION AND LANE USE GUIDE SIGN...32 FIGURE 13: COMBINED DESTINATION AND LANE USE REGULATORY SIGN...32 LIST OF APPENDICES APPENDIX A SAMPLE PUBLIC INVOLVEMENT MATERIAL APPENDIX B MDOT ROUNDABOUT QUICK GUIDE APPENDIX C EXAMPLE ROUNDABOUT MONITORING REPORT iii

5 SECTION 1: INTRODUCTION Section 1.1: Content and Applicability of Guide This document is intended to assist roundabout designers and is for use by MDOT staff as they study, design, and build modern roundabouts within the state of Michigan. The principles and direction identified in this document should be used for all roundabouts being planned, designed, or constructed on routes under MDOT s jurisdiction. This document is a supplement to the FHWA document Roundabouts: An Informational Guide (publication number FHWA-RD ) which can be viewed and downloaded at the following web address: Throughout this document, the FHWA Guide is cited and other supplemental information is provided where relevant. This document is not intended to serve as a comprehensive and rigid set of design standards. Rather, it provides general guidance and identifies important principles and considerations related to some roundabout design issues. Because this is the first roundabout guide to be prepared by MDOT, it is general in nature with specific information provided in only a small number of cases. It is MDOT's intention to supplement and revise this document over time, including development of roundaboutspecific information in the MDOT Design Manual. This may include information related to geometric design, pavement markings, landscaping, lighting, and signing. In addition, this document does not address all of the specific situations which could arise during the course of roundabout design. When unique situations arise, designers are encouraged to reference the FHWA Guide and consult with experienced roundabout designers. This document contains information regarding roundabout planning, safety, geometric design, pavement markings and signing, public involvement, and other design/operational considerations. Within each of these sections, numerous topics and issues are addressed. For each of the topics/issues, guidance is provided regarding main goals and objectives, the location of relevant standards (when available), and related factors that need to be considered. Rather than repeat information that is included elsewhere (e.g., the FHWA Guide, American Association of State Highway and Transportation Officials (AASHTO) green book, etc.), citations are provided so that users can obtain information from each source. Section 1.2: Basic Terminology Throughout this document, some basic terminology is used. The most common terms used in this document are defined below. In addition, Figure 1 shows some of these common terms as they relate to an actual roundabout. A single-lane roundabout is a roundabout with one entering lane per approach. Two-lane roundabouts have at least one entry with two lanes separated by pavement markings, and some or all of the circulating road usually has two lanes. Three-lane roundabouts have at least one entry with three lanes, and some or all of the circulating road usually has three lanes. A bypass lane or right turn bypass lane is typically used to accommodate heavy right turn movements to improve the capacity of a roundabout. The bypass lane is typically separated from an entrance by a curbed island and allows right-turning traffic to avoid entering the roundabout. Bypass lanes can either be free flowing or yielding to exiting traffic (semi-bypass lane). 1

6 SECTION 1: INTRODUCTION Figure 1: Common Roundabout Terminology There are six geometric parameters that significantly affect the capacity of a roundabout (Figure 2). These parameters are based on effective geometry measured from curb face to curb face. For more detailed information about these parameters, how they are measured, and their effect on capacity, see the Rodel User s Manual (Rodel Software Limited, 2007). They include the following: Entry Width (E) Width of the roadway where it enters the roundabout. Entry width is measured perpendicularly from the outside curb face to the inside curb face at the splitter island point nearest to the inscribed circle. Flare Length (L') The distance over which the approach roadway widens to the entry width, if such flaring is present. Half Width (V) The width of the approach roadway (existing roadway) before it starts to widen, if flaring is present. Entry Radius (R) The radius of the outside curb at the entry. Entry Angle (PHI) (Φ) The mean angle at which entering and circulating traffic cross paths. Inscribed Circle Diameter (D) This parameter is also abbreviated as ICD. This is the outside diameter of the roundabout. 2

7 SECTION 1: INTRODUCTION V Figure 2: Geometric Parameters Pedestrian crossings are typically located one car length before the yield line and are usually painted crosswalks. Pedestrian crossings allow pedestrians to cross one direction of vehicle travel at a time and provide median refuge in the splitter island. The circulatory road is the curved roadway around the central island where vehicles have the right-ofway (ROW) and travel in a counterclockwise direction. Mini-roundabouts are small roundabouts used in low-speed urban locations with speed limits of 35 mph or less. These are not addressed in this document. Double roundabouts are two closely spaced (usually less than 100 feet apart) roundabouts connected by a short roadway link. Spiral pavement markings are the pavement markings used for multilane roundabouts to keep vehicles in the correct lane while entering, circulating, and exiting the roundabout. They are called spiral because they guide traffic from within the roundabout out the exits without changing lanes. Spiral hatching is a cross-hatch pavement marking used to shift traffic in the circulating road of a roundabout outward into an adjacent lane. This treatment is used in uncommon situations such as double left turns and when applied correctly enhances safety and traffic operations. Splitter islands are raised islands used to separate approaching and exiting traffic. These islands are also used as a pedestrian refuge. The central island is the raised island defined by curb in the center of the roundabout. The fastest path is the shortest possible route a single vehicle can travel through a roundabout in the absence of other traffic and ignoring all lane markings. The fastest path determines the fastest possible entering, exiting, and circulating speeds within a roundabout. Path overlap occurs when the natural path of a vehicle traveling through a roundabout overlaps the path of an adjacent vehicle. See Section 6.45 of the FHWA Guide. A truck apron is the paved portion of the central island located adjacent to the circulating roadway. It is defined by mountable curb on the outside and helps accommodate large trucks. 3

8 SECTION 2: PLANNING Section 2.1: Introduction Roundabouts should be considered as one potential intersection option within MDOT-sponsored or funded planning studies/design projects since they offer improved safety, cost savings, and enhanced traffic operations in many situations. This includes freeway interchanges where intersections currently exist or would be created at ramp terminals. A comparison of roundabout practicality/feasibility vs. other intersection types should be conducted, taking into consideration safety, traffic operations, capacity, ROW impacts, and cost. Other factors as described below can also be included in the evaluation if desired and deemed appropriate. When conducting such comparisons, roundabouts are not the optimal solution in all cases, but offer significant benefits in many situations. Additional details regarding many of the topics discussed in this section are provided in subsequent chapters of this document. An evaluation matrix template was developed in order to aid in the design and decision making process. This matrix can be used for safety, scoping, and Early Preliminary Engineering (EPE) studies and is located in the MDOT Roundabout Quick Guide (Appendix B). The MDOT Roundabout Quick Guide is intended to assist with the planning and evaluation process once the decision is made to consider a roundabout option. Section 2.2: General Considerations Planning Process Planning for roundabouts entails consideration of a wide range of factors. A typical planning process would include: 1. Data collection 2. Development of 20-year traffic projections 3. Capacity analysis using Rodel software (Rodel is a computer software program designed specifically to analyze traffic operations and geometry at roundabouts. It is recognized by most roundabout designers as the best model for this purpose and is widely used in the United States) 4. Preparation of a roundabout concept design 5. Public involvement 6. Comparison to other intersection types 7. Documentation (report or memo) 8. Selection of preferred option The goal of the planning process is to make a sound decision regarding whether a roundabout is feasible, whether it is a better solution than other intersection types, and whether it should be advanced into the preliminary design phase. As would be the case for any type of road design project, early and ongoing coordination with the MDOT Geometrics Design Unit (MDOT GDU) should be carried out throughout the duration of the project at key milestones. Chapters 2 and 3 of the FHWA Guide provide additional information regarding planning for roundabouts. 4

9 SECTION 2: PLANNING Data Requirements Data that is typically required in order to evaluate a roundabout would include the following: Existing AM and PM peak hour turning movement counts If present, major traffic generators with shift changes that occur during off peak hours MDOT-approved design year (i.e., 20-year) AM and PM peak hour turning movement projections Design vehicle to be accommodated Base mapping (either aerial photograph, aerial mapping, or survey) ROW mapping Crash data for the most recent three-year period available Location of nearby intersections and signal timing information (if applicable) Location of any major constraints near the intersection (i.e., ROW, major utilities, structures, railroad crossings, or water bodies) Existing and future planned bicycle and pedestrian facilities Truck percentages Data that is desirable to obtain, though not necessarily required in all situations, includes: Existing pedestrian counts Previously prepared construction plans or as-built plans showing the existing intersection(s) Utility information Evaluation Criteria In assessing the desirability of a roundabout relative to other intersection types, evaluation criteria would typically include some or all of the following factors, as relevant: Safety Capacity Traffic operations Cost Design life (normally 20 years, less than this is undesirable) ROW impacts Accommodation of pedestrians and bicyclists Aesthetics Proximity to other intersections Driveway accommodation and access management opportunities Public input Constructability 5

10 SECTION 2: PLANNING Maintaining traffic Social, Economic, and Environmental impacts Please note that most versions of Rodel software (which is used for MDOT s roundabout evaluations) report roundabout delay as stop delay. Stop delay includes only the time when a vehicle is actually stopped while waiting to enter an intersection. Most software that is used to evaluate intersections controlled by traffic signals reports delays in the form of control delay. Control delay includes both stop delay and geometric delay which is the time that is lost as a vehicle decelerates while approaching an intersection, maneuvers through the intersection, and accelerates away before reaching its original speed. In cases where control delay is used/reported for an intersection with a traffic signal or stop control, control delay for a roundabout at the same intersection can be calculated by adding the geometric delays found in Table 1 to the stop delay reported from Rodel (all versions of Rodel except version report stop delay version reports geometric delay, so the geometric delays in Table 1 should not be added to the delay reported from version ). At roundabouts, the size of the Inscribed Circle Diameter (ICD) has little effect on geometric delay. The approach speed is more important, because the extra distance required to travel around a larger ICD is essentially offset by the faster circulating speed. When comparing traffic operations of a roundabout concept against other intersection types, the main criteria considered should be average seconds of control delay rather than level of service (LOS). LOS can be provided for informational purposes if desired. Control delay should be used when conducting cost/benefit analysis. Table 1: Geometric Delay For Roundabouts Road Approach Speed (MPH) Average Geometric Delay per Vehicle (add to Rodel delay to get control delay) 30 9 seconds seconds seconds seconds When evaluating a roundabout with other intersections nearby, it is extremely important to assess the interaction of the intersections. Chapter 8 of the FHWA Guide provides additional information on this topic. This assessment should take into consideration queue lengths, lane utilization, the distance between the intersections, arrival and departure patterns (i.e., random versus platoons), and potential changes to signal timing. General Capacities One common question that arises regarding roundabouts is the relative capacity of single-lane, two-lane, and three-lane roundabouts. As discussed in more detail in Section 4 of this document, a roundabout s capacity is determined based on its geometry and peak hour traffic volumes/turning patterns. Because geometry and peak hour traffic volumes can vary considerably within each roundabout category (i.e., single-lane, two-lane, three-lane), it is not possible to develop a precise capacity that would apply to each category. However, Table 2 presents approximate maximum capacities (total hourly entering volumes from all approaches) for each category. It should be noted that the capacities provided in Table 2 are only a general guide, and there is no substitute for an intersection-specific capacity analysis using Rodel 6

11 SECTION 2: PLANNING software. Furthermore, designers should keep in mind that the general capacities provided in Table 2 may not be accurate in all individual circumstances. Table 2: Typical Roundabout Capacities Type of Roundabout Single-lane Two-lane Three-lane Approximate Peak Hour Capacity (Combined entering volume for all approaches) Up to 2,000 vehicles per hour Up to 4,000 vehicles per hour Up to 7,000 vehicles per hour Section 2.3: Beneficial Locations and Applications Implementation of roundabouts can be beneficial to the traveling public in a wide variety of situations. The list that follows below identifies some of the most common locations and/or applications where installation of a roundabout may be advantageous. However, designers and decision-makers need to recognize that this list is general and will not apply in all situations. There are undoubtedly useful applications of roundabouts that are not included in this list, and under some circumstances, applications included on the list may not be appropriate.. Site-specific analysis of the feasibility of a roundabout should be conducted at each individual location. High-speed rural intersections: Studies and experience in the United States and other countries show that roundabouts are an exceptional safety countermeasure at these locations. Other states that have installed roundabouts at high-speed rural locations have reported very dramatic reductions in total crashes, injury crashes, and fatal crashes. This is consistent with the experiences of other countries around the world. Intersections with crash histories: Studies and experience show that roundabouts can result in very significant reductions in crashes, especially injury crashes and fatal crashes. The most notable types of crashes which can be reduced include left turn head-on and angle crashes. Intersections with traffic operational problems: Properly designed roundabouts can be a very effective tool in eliminating congestion and delays. Closely spaced intersections: Roundabouts can eliminate traffic queuing from one intersection into another. They can also eliminate problems related to coordination of traffic signal timing between closely spaced intersections. Intersections near structures: In most situations, roundabouts do not require as many approach lanes as signalized intersections for vehicle storage. In situations where a bridge structure is located near an intersection, installing a roundabout can allow the use of a shorter or narrower bridge structure, resulting in significant cost savings. The most common situation in this category is at freeway interchanges. Freeway interchanges: For the reasons noted in the preceding bullets, roundabouts can be beneficial at ramp terminals of freeway interchanges. In addition, vehicles exit a roundabout randomly spaced which can be beneficial as they merge from an on-ramp into the stream of traffic on a freeway mainline (this is similar to the effect achieved through ramp metering in congested urban areas). As a part of an access management program: Because roundabouts can accommodate U-turns, they can be implemented as a part of an overall access management plan, especially at intersections that display other characteristics that make roundabouts desirable such as crash problems or traffic 7

12 SECTION 2: PLANNING operational problems. In these situations, a roundabout can function in much the same way as a median turnaround. Intersections with unusual geometry: Because roundabout geometry is relatively flexible, some intersections with unusual geometrics can be improved by the installation of a roundabout. Section 2.4: Locations and Applications Where Care Should Be Exercised As might be expected, there are also locations and applications where roundabouts may not be beneficial, and care should be exercised when considering a roundabout in these situations. Similar to the preceding section which identified beneficial applications, the list provided below is general and will not apply in all situations. Furthermore, at some locations, a roundabout may be a good solution even though the location has some of the characteristics noted below. Again, case-by-case analysis is required, keeping in mind that some of the circumstances listed below would also cause potential concerns at intersections with traffic signals. Intersections within a system of coordinated signals: Within corridors that contain multiple traffic signals having relatively good progression, a roundabout may not be the best overall solution. In these situations, a roundabout may result in the disruption of traffic platoons. Intersections with steep grade: At locations where approaches and/or an intersection has a steep grade (i.e., five percent or greater) care should be taken. It would usually be undesirable to construct a roundabout where the grade running through the intersection is greater than five percent. Potential concerns in these situations would include the ability for drivers to stop when roads are snow-covered or icy, as well as the potential for trucks to tip over. Note: this would apply to both roundabouts as well as intersections with traffic signals. Designers should note that it might be reasonable to construct a roundabout at locations where approach grades are above five percent if the intersection and approaches within about 100 feet can be designed at five percent or less. Intersections where stopping sight distance cannot be achieved: Drivers must be able to identify an intersection and have adequate time to stop if necessary. This concern could be due to either a vertical or horizontal curve (with or without superelevation). This would apply to all intersections (roundabouts and other types). Intersections near railroad crossings: If a roundabout is being considered near a railroad crossing, careful evaluation should be conducted to ensure that traffic will not queue from the roundabout onto the railroad tracks or vice versa. Traffic queuing from a railroad crossing into the circulating road of a roundabout can result in gridlock with the result being that motorists cannot enter or exit the roundabout from any direction. Closely spaced intersections: Similar to intersections near railroad crossings, care needs to be taken to ensure that adjacent intersections will not back up traffic into a roundabout. This is especially important where a roundabout is installed to mitigate an existing congested intersection. In these circumstances, the roundabout can usually process traffic more efficiently than the previous intersection, with the result being that downstream intersections could back up traffic into the roundabout. 8

13 SECTION 3: SAFETY Section 3.1: Introduction In addition to operational benefits noted in Section 2, safety benefits of properly designed modern roundabouts are very significant (Insurance Institute for Highway Safety, 2000; Lalani, 1975). This section of the document provides general direction regarding the relationship between geometric design and safety. It also provides some information regarding how roundabouts can be considered as a potential safety countermeasure. To provide some perspective regarding the potential for safety improvements at Michigan intersections, crash data for 2005 is typical and shows the following for the state of Michigan: Approximately 30 percent of all crashes were at intersections. About 29 percent of all injuries occurred at intersections. About 26 percent of all fatalities occurred at intersections. United States studies have shown that relative to other intersection types, roundabouts typically reduce overall crashes by approximately 40 percent, reduce injury crashes by approximately 75 percent, and reduce serious injury and fatal crashes by about 90 percent (Insurance Institute for Highway Safety, 2000). Thus, the potential for improved intersection safety in Michigan is substantial. Figure 3 shows the differences in the number and type of conflict points between a standard intersection and a roundabout. In addition to reducing the number of conflict points, a roundabout also eliminates the types of conflicts that typically result in the most serious crashes (i.e., left turn head-on crashes and angle crashes). In addition, typical speeds within a roundabout range between 10 and 25 mph. As a result, crashes that do occur usually have a lower severity than those at other intersection types. Figure 3: Conflict Points at a Standard Intersection and a Roundabout. 9

14 SECTION 3: SAFETY Designers should also keep in mind that single-lane roundabouts typically provide a greater crash reduction than multilane roundabouts. Crash frequencies and rates at multilane roundabouts will at times be comparable to traffic signals. However, crash severity remains notably lower on average than traffic signals. As a result, multilane roundabouts still provide considerable safety benefits relative to other intersection types. Section 3.2: Geometric Design and Safety Section 4 of this document provides information regarding geometric design (including information on sight distance, grades, cross slopes, etc.). Following the guidance provided in that section is extremely important in order to ensure the safest possible geometric design. The FHWA Guide provides additional information regarding roundabout safety in Chapters 5 and 6. Here are some general principles regarding geometry that will maximize roundabout safety in most instances: Minimize entry/circulating road widths, ICD, and number of lanes. Keep entry and exit radii within the appropriate range (i.e., avoid very small or very large radii). Control vehicle speeds within an acceptable range (based on roundabout type and speed) along the fastest path prior to the yield line (discussed in more detail in Section 4 of this document and Chapter 6 of the FHWA Guide). Keep the entry angle for each entrance within the appropriate range (typically between 20 and 35 ). Where practical, increase capacity by using a longer flare length as opposed to a wider entrance. Maximize the angle between adjacent legs of the intersection. Avoid entry and exit path overlap at multilane roundabouts. Designers need to understand while these principles apply in most situations, there are exceptions and circumstances where they would not apply. For additional details about these principles and their application, see Accidents at 4-Arm Roundabouts (TRL Laboratory Report 1120, 1984). Section 3.3: Roundabout Use as a Safety Countermeasure Evaluation Process Evaluating a roundabout as a potential safety countermeasure can be accomplished in different ways and need not be an overly burdensome process. Evaluation methods may include a Cost/Benefit (C/B) Ratio or examination of existing crashes versus anticipated reductions based on roundabout safety studies. For additional information regarding the C/B ratio, see the MDOT Roundabout Quick Guide. The potential safety benefits of a roundabout can also be evaluated by identifying the crash frequencies and types presently occurring at an intersection and determine whether these would likely be eliminated by a roundabout, assuming that average crash reductions would be realized. The crash types that are of the most concern at any intersection are those that result in serious injuries and fatalities. Typically, left turn head-on crashes and angle crashes are the most dangerous types at intersections, and both of these crash types are essentially eliminated by installation of a roundabout. In addition, high-speed intersections (i.e., approach speeds of 45 to 65 mph) are likely to experience a significant safety benefit since crash severity is often high at these intersections. Roundabouts in other states have been shown to be a very effective safety countermeasure at high-speed intersections. However, implementation of roundabouts should not be limited only to intersections experiencing head-on and angle crashes since 10

15 SECTION 3: SAFETY other types of crashes are also reduced or eliminated when roundabouts are installed. By collecting and analyzing at least three years of crash data (five years is preferable, if available), crash frequencies, patterns, types, and severity can be identified. Once this information is analyzed, reasonable judgment can be exercised regarding whether a roundabout would be likely to reduce crashes. If another type of intersection control (i.e., traffic signal, four-way stop, two-way stop) is being considered and compared to a roundabout, typical crash frequencies, severities, and rates can usually be estimated based on the performance of the same intersection control type at other existing intersections in the region. For example, in southeast Michigan, the Southeast Michigan Council of Governments (SEMCOG) publishes average crash information for various types of intersections within its jurisdiction (SEMCOG, 1997). Throughout the evaluation process, care should be taken to ensure that variables other than intersection type are not substantially contributing to crashes. For example, consider an intersection where crashes are largely due to inadequate forward stopping sight distance on an approach. Installing a roundabout without rectifying the sight distance problem would be unlikely to satisfactorily address the situation. Follow-up Monitoring At locations where roundabouts are constructed, follow-up monitoring should be conducted periodically to determine safety performance after implementation. At most locations, it would be prudent to exclude from consideration data for the first three to 12 months of operation, as it would be expected that motorists are adjusting to the new intersection during this time frame. In some situations, adjustments to pavement markings and signing may be warranted based on crash patterns after implementation. The applicable MDOT Transportation Service Center would normally be responsible for operations and monitoring at a roundabout located within their service area. 11

16 SECTION 4: GGEOMETRIC DESIGN Section 4.1: Introduction This section of the document contains information that is relevant to the geometric design of roundabouts. Topics included in this section include basic geometric elements, capacity analysis, general guidance, three-lane roundabouts, trucks, cross slope, sight distance, and pedestrians and bicyclists. Details regarding most of the topics discussed in this section can be found in the FHWA Guide (Chapter 6) and the Rodel User s Manual (Rodel Software Limited, 2006). Therefore, this section of the document provides general direction, but relies on these other sources for specific information. Four main criteria are considered when determining roundabout geometry. These broad categories are the capacity analysis, safety-related requirements, cost, and site constraints. In some cases, these factors can conflict with each other, resulting in the need for designers to assess trade-offs and identify the optimal balance for a particular design. When developing a roundabout design, the designer would initially develop a preliminary concept. The concept is then refined based on input from reviewers and stakeholders both within and external to MDOT, as applicable. Once this concept design is complete, base plans would be developed followed by preliminary and final plans. This section of the document does not address or dictate the format, layout, content, or details regarding plan sets. As would be the case for any type of road design project, early and ongoing coordination with the MDOT GDU should be carried out throughout the duration of the project at key milestones. Section 4.2: Geometric Elements The most important geometric elements at a roundabout include ICD, entry width, entry radius, entry angle, flare length, half width, central island, circulating road width, splitter islands, exit width, and exit radius (See Figures 1 and 2). Of these, the first six have been identified as having a significant effect on roundabout capacity (these are discussed in the capacity analysis section below), and are considered when determining capacity. At some roundabouts, right turn bypass lanes are also utilized. Collectively, these geometric features should provide adequate capacity to ensure safe and efficient traffic operations. Typical ranges for these geometric elements are provided in the FHWA Guide as well as the Rodel User s Manual. Section 4.3: Capacity Analysis A capacity analysis is required for each specific roundabout prior to concept design. To determine the required geometry and corresponding queues and delays for each roundabout, it is essential to conduct a capacity analysis. Rodel software should be used to analyze roundabout capacity and determine roundabout geometry in all cases. Except in unusual circumstances, MDOT-approved 20-year traffic projections (AM and PM peak hour) must be used for the capacity analysis. For details regarding Rodel software and its use, please see the Rodel User s Manual. When conducting a capacity analysis, geometric parameters are adjusted in Rodel through an iterative process (i.e., numerous adjustments) to achieve the desired delays and LOS in each peak hour. During this optimization process, the designer needs to keep in mind site constraints (major issues with regard to ROW, nearby bridges, utilities, etc.) and other roundabout design principles related to speed control. For additional information regarding roundabout capacity, see the Rodel User s Manual and the Transport and Road Research Laboratory Lab Report 942, The Traffic Capacity of Roundabouts (TRL 1980). 12

17 SECTION 4: GEOMETRIC DESIGN Lane Balance Lane balance and utilization is tested at multilane roundabouts for both peak hours after the geometry has initially been identified. By default, the current version of Rodel assumes equal utilization of all entry lanes at a multilane roundabout. In some situations, incorrect lane assignments (i.e., right, through, left) will affect lane utilization enough to result in significant unbalanced lane use, long delays, and long queues. Therefore, once roundabout geometry is identified at multilane roundabouts, it is important to analyze lane usage by manipulating the capacity factor function in Rodel. This will result in identification of proper lane assignments and should be reflected in the concept design. Users need to toggle from the flow factor to the capacity factor by using the F4 key to test lane balance and identify lane assignments. Once the capacity factor has been enabled, this value should be changed from the default 1.00 to 0.50 (two-lane entry) or 0.33 (three-lane entry) for the leg to be analyzed. This allows the capacity of one lane to be tested with the peak hour traffic volume for a specific turning movement (i.e., right, through, left). The movement to be analyzed must be isolated by zeroing out the other two movements. If the predicted queues and delays for the movement are acceptable using one lane, then the designer can either assign the lane only for that movement (e.g., left only, right only, etc.) or as a combined use which includes that movement (e.g., left/through, etc.). More than one lane may be needed for the movement (e.g., double left, etc.), if queues and delays are not acceptable. This process can be repeated for each movement and each leg to determine lane assignments for the intersection. Based on these results, the designer can adjust geometry and pavement markings. Bypass Lanes A bypass lane allows vehicles to circumvent a roundabout, providing additional capacity. They are used mainly in situations where a large portion of turning movements are right turns. Typically, bypass lanes should only be used when other geometric layouts fail to provide acceptable traffic operations, and the decision to use bypass lanes should take into account pedestrian and ROW constraints. In some cases, bypass lanes provide significant benefits. Two types of bypass lanes may be used. The first is a free flow bypass lane which allows vehicles to bypass the roundabout and then merge into the exiting stream of traffic. The second type is a semi-bypass lane which requires approaching vehicles to yield to traffic leaving the adjacent exit. For more details on this topic, see Section of the FHWA Guide. Confidence Level Capacity analyses are initially conducted at the 50 percent confidence level setting in Rodel. Once acceptable geometry has been identified, the 85 percent confidence level analysis is then conducted, and geometry is adjusted if necessary (the 85 percent confidence level assumes below average capacity and builds in a reasonable safety buffer). Each entrance should function at LOS D or better (LOS B or better is preferred when possible) at the 85 percent confidence level. If this process shows that an entrance is predicted to operate at worse than LOS B at the 85 percent confidence level, modified geometry should be assessed in Rodel in an attempt to achieve LOS B or better without adding unacceptable costs or negative impacts. The geometric elements most likely to address this problem are flare length (L), right turn bypass lanes, entry width (E), and inscribed circle diameter (ICD). In most cases, designers should first consider increasing the flare length. If this is not successful, a wider entry (either sub-lane widening or adding an extra entering lane), a bypass lane, or larger ICD may be needed. All three of these geometric changes have the potential to increase costs and ROW impacts, so this must be weighed against the risk of slightly worse operations at the subject approach. Note that the 50 percent confidence level results should be used in reporting queues, delays, and LOS and for comparison to other intersection alternatives being considered (i.e., traffic signals, stop control). 13

18 SECTION 4: GEOMETRIC DESIGN Because Rodel was developed based on measurements of roundabout capacity in the United Kingdom, there is some question as to whether roundabouts in United States will achieve similar capacities. Limited information from around the United States seems to indicate that there may be an initial period of lower capacities, but overall, capacity, queues, and delays appear to be similar to those predicted using Rodel. Evaluating each roundabout at the 85 percent confidence level is a conservative approach that assures acceptable queues and delays, even if the roundabout experiences capacity below that predicted by Rodel at the 50 percent confidence level. In addition, at most intersections, opening day traffic volumes are considerably lower than predicted 20-year flows. Where this is the case and geometry is designed to accommodate 20-year flows, an initial period of reduced capacity is unlikely to have any meaningful effect on queues or delays. However, care should be exercised in situations where design volumes could be reached early in the life of the roundabout, especially in situations where traffic volumes may be artificially suppressed due to long delays at the present intersection or network. Peak Hour Factor Rodel does not use a peak hour factor when calculating queues and delays. Instead it uses the flow ratio parameter to represent the rise and fall of traffic during a peak hour. In most cases, the default flow ratio should be used. However, certain unique situations with very high or very low peak hour factors may warrant adjustment of the flow ratio to more accurately represent the peak within the peak hour. There is no uniformly accepted method for converting between peak hour factors and flow ratios, so designers should coordinate with the MDOT GDU for direction when there may be the need to deviate from the default values in Rodel. Diameter The ICD used in Rodel for the capacity analysis must meet criteria for speed control (this is discussed in more detail below). Effective Width The geometry used in Rodel is measured curb face to curb face (i.e., effective width). Single Lane Entries Designers should exercise care when evaluating single-lane roundabouts using Rodel when entries are going to be wide (18 to 20 feet) to accommodate trucks. Rodel assumes these widths represent two narrow entry lanes instead of one wide lane. When only one entering and one circulating lane are actually present at these widths, this results in over prediction of capacity and under prediction of delays and queues. Therefore, single-lane entries should be modeled in Rodel at 15 feet or less with 13 to 14 feet being more conservative yet. This capacity analysis procedure is reasonably conservative and should be used even when actual entry geometry is designed wider to accommodate trucks. Pavement Markings and Capacity The United Kingdom roundabouts that were studied to develop the capacity prediction equations which underlie Rodel usually did not contain lane assignment arrows on the approaches or pavement markings delineating lanes in the circulating road. However, most multilane roundabouts in the U.S. do incorporate these markings. These markings will in most cases slightly increase capacity and decrease delays/queues relative to what is predicted by Rodel. However, these differences are relatively minor and are not quantifiable. For these reasons, a designer should not change geometry in any way based on this assumed increase in capacity. Similarly, designers should not assume any changes to the delays and queues predicted by Rodel. 14

19 SECTION 4: GEOMETRIC DESIGN Maximum Queue The maximum queue reported by Rodel is the largest total number of vehicles queued on an approach (in all lanes added together) at the end of any one time slice during the results period (users should see the Rodel user s manual for more details on maximum queue, times slices, and results period). The maximum queue is calculated using the 50 percent confidence level. Assuming balanced lane use is achieved, the maximum queue in any one lane would theoretically be equal to the maximum queue divided by the number of lanes. Because the maximum queue is defined as the longest queue at the end of the time slices, it is influenced by the number and length of the time slices which are defined by the user. If the default values are changed to include more/shorter time slices, the maximum queue will usually increase because the default 15-minute time slices can mask some of the variation within each time slice. In situations where queue length is a key issue, detailed queue analysis using times slices of one to five minutes is advisable (queue evolution can be viewed in version of Rodel using the F6 key). It should also be noted that the maximum queue is quite different from the 95 percent random queue. The 95 percent random queue is determined by the random variation around the average queue and can be considerably longer than the maximum queue. While it may be longer than the maximum queue, the 95 percent queue only occurs five percent of the time, meaning that it takes place approximately fifteen days per year, usually for only a few minutes during the peak hour. The maximum queue is one important piece of information, but the longest queue which is actually observed during a peak hour can be considerably longer than the maximum queue predicted by Rodel. This is due to random variation and the use of 15-minute time slices as noted above. In order to minimize the potential for queuing problems, designers should assume that the worst actual observed queue during any peak hour could reach up to two times as long as the maximum queue predicted by Rodel. This is especially important when a roundabout is being designed close to an adjacent intersection or where a queue on a freeway off ramp could potentially back onto the freeway mainline. In some unusual circumstances (e.g., special event traffic, holiday weekend traffic, detoured traffic from another route, etc.), the longest observed queue could be longer than two times the maximum queue. Pedestrians and Trucks High volumes of pedestrians and trucks can reduce the capacity of roundabouts. Truck percentages should be represented by modifying the Passenger Car Units (PCU) field in Rodel (see the Rodel User s Manual for details). In situations with relatively high pedestrian volumes, the effect on capacity is assessed by using the pedestrian capacity reduction factors noted in Exhibits 4-7 and 4-8 of the FHWA Guide. These factors can be entered into the capacity factor field of Rodel for each leg of the roundabout. Simulation Tools There are a number of simulation tools available to visualize the operation of roundabouts. All of the simulation software programs presently available use gap acceptance theory to determine the capacity of each roundabout entry and therefore do not accurately represent capacity, delays, and/or queues. As a result, these programs should be used with great caution (if at all) for analysis. Although they do not accurately portray traffic operations, they can be an effective tool for showing general roundabout traffic operations to the public. 15

20 SECTION 4: GEOMETRIC DESIGN Section 4.4: General Design Guidance Speed Control Roundabout geometry is used to control speeds on the approaches, at the entries, within the circulating road, and at the exits. How, where, and to what degree speeds are controlled has a direct effect on the safety and traffic operations of a roundabout. There are generally two different speed regimes that need to be considered at multilane roundabouts. First, during peak traffic times, motorists usually stay within their chosen lanes. Second, during off-peak times, some motorists will ignore lane markings and travel along the shortest path possible (dictated by the roadway center line and curb locations). This is typically called the fastest path. At single-lane roundabouts, the only speed regime that is typically considered is the fastest path. Chapter 6 of the FHWA Guide provides information regarding speed control which is essential to proper roundabout design. Figure 4 is a diagram showing the three different vehicle paths (through movement, left turn, and right turn) at a roundabout. The radii on each of these vehicle paths control the respective speeds. As a general rule, the vehicle speed associated with the radius on the approach (R1) should be within 12 mph of the speed associated with the radius of a circulating vehicle (R4). Table 3 of this document (Table 3 in this document is the same as Exhibit 6-14 in the FHWA Guide) provides the maximum allowable R1 values for single-lane and two-lane roundabouts of various diameters. Table 3: Approximate R 4 Value and Maximum R 1 Value Inscribed Circle Diameter (ICD) Approximate R 4 Value Maximum R 1 Value Radius (ft) Speed (mph) Radius (ft) Speed (mph) Single-lane Roundabout Double-Lane Roundabout Source: FHWA Guide (Exhibit 6-14). 16

21 SECTION 4: GEOMETRIC DESIGN Figure 4: Vehicle Path Radii (Source: FHWA Guide) The fastest path for a roundabout concept design should be drawn using the spline function in a CAD computer program (the spline function creates a continuous smooth curve using vertices as control points, allowing designers to simulate a true vehicle path using straight lines and arcs usually does not replicate an actual vehicle path). Directions regarding how to draw the path in relation to center lines and curb face locations are provided in Chapter 6 of the FHWA Guide. Designers should note that there is one change that is needed in the FHWA Guide regarding this topic. Exhibit 6-6 shows a fastest vehicle path for a two-lane roundabout with the offset on the approach from the lane line which is incorrect. Instead, the offset on the approach should be from the center line or from the curb face of a boulevard median island, if applicable. When measuring R1 values, an arc that is between 65 and 80 feet long should be drawn on top of the spline within 150 feet of the yield line to represent the R1 radius. The arc should be located at the location of the smallest radius within 150 feet of the yield line. When preparing a roundabout concept design, it would not be unusual to go through multiple iterations of approach geometry for each leg of the roundabout while trying to achieve proper speed control. For maximum R1 values, see Table 3 above. By following this guidance and achieving enough curvature (also known as deflection ) prior to the yield line (i.e., R1 is below the maximum values in Table 3), the potential for entry-circulating crashes is usually minimized. However, designers should not reduce R1 excessively (i.e., below approximately 100 to 150 feet, which corresponds to approximately 20 to 24 mph) because doing so usually increases the occurrence of single vehicle loss of control crashes on the approaches. Put another way, the net safety benefit is unlikely to be maximized with R1 values below 100 to 150 feet. In addition, over deceleration caused by very small R1 values will usually reduce capacity and can cause entry path overlap at multilane roundabouts. Path Overlap At multilane roundabouts, entry path overlap occurs when the natural path of one traffic stream through the roundabout overlaps the natural path of another traffic stream. This phenomenon will reduce capacity and also creates safety concerns. Sections and of the FHWA Guide provide information regarding this issue. Achieving proper entry geometry at multilane roundabouts is particularly challenging because the designer must control R1 values while not causing path overlap, and these two 17

22 SECTION 4: GEOMETRIC DESIGN objectives usually conflict with each other. However, proper entry geometry is extremely important to the proper function of multilane roundabouts, and requires great attention to detail. General Geometry Ranges There are some general ranges of widths and radii that are commonly used for roundabout geometry. Please note that these are general ranges only and are not rigid standards or rules. There will be exceptions to these ranges from time to time due to specific circumstances at individual intersections. Therefore, designers are encouraged to carefully evaluate each situation on a case-by-case basis, remembering that there is no substitute for experience. Inscribed Circle Diameter Single-lane roundabout: 120' 170' Two-lane roundabout: 145' 200' Three-lane roundabout: 210' 250' Compact urban roundabout: 90' 120' (normally for roads with posted speeds of 30 mph or less) Entry Radius Single-lane roundabout: 50' 80' Two-lane roundabout: 70' 100' Three-lane roundabout: 80' 120' Exit radii are often within similar ranges, but are sometimes substantially larger. Entry Widths (Curb Face to Curb Face) Single-lane roundabout: 15' 20' Two-lane roundabout: 22' 28' Three-lane roundabout: 33' 40' Exit widths are usually similar or identical to these entry widths. Circulating road widths are usually 1.0 to 1.2 times the width of the widest entry (curb face to curb face) Single-lane roundabout: 18' 25' Two-lane roundabout: 30' 34' Three-lane roundabout: 42' 48' Splitter Islands Geometry for splitter islands is described in Section of the FHWA Guide. Lane Drop Tapers At multilane roundabouts, it is common to drop one travel lane at an exit. Vehicles leaving a roundabout are typically traveling between 15 and 25 mph and are accelerating away from the intersection. When designing a lane drop at a roundabout exit, the taper should not start until after the exit radius is complete. There should also be a short parallel section before beginning the taper if practical. Exit tapers should be designed assuming a 35 mph design speed using the formula for lane drops at less than 40 mph. Under some unusual circumstances, using a different design speed may be applicable. In these cases, consult with the MDOT GDU. Designers should also consider which lane would be dropped (i.e., inside lane or outside lane). It is recommended that the right (outside) lane should be dropped. However, designers should evaluate the likely traffic volumes exiting the roundabout on each individual exit lane. If there is a 18

23 SECTION 4: GEOMETRIC DESIGN substantially heavier traffic volume that would be using the outside lane, it may be beneficial to drop the inside lane instead. Again, consultation with the MDOT GDU is recommended. When a free flow right turn bypass lane is utilized, the design of the merge should follow the MDOT Design Manual which takes into account the relative speeds of the two conflicting streams of traffic and provides the necessary lengths for the parallel section and merge section. Lane Consistency When preparing a concept design, it is important that the designer considers lane consistency at each potential combination of entry, circulating, and exit lanes. As a general principle, there must always be lane consistency throughout the entire intersection. Although it may seem obvious, there should never be a situation where two lanes merge into one within the intersection, and once a driver selects the proper lane before entering the roundabout, they should be able to stay in the same lane as they circulate and exit. Achieving lane consistency includes aspects of both geometric design and pavement markings (pavement markings are discussed in more detail in Section 5 of this document). It is recommended that a simple hand sketch or diagram be prepared prior to actually drawing a roundabout using a CAD program. When preparing this sketch, lane assignments (i.e., left only, right only, through only, combined through/left, combined through/right) must be included in order to be able to track each movement through the intersection and out the exits. In addition, the number of circulating and exiting lanes at each part of the intersection can be confirmed when preparing this sketch. This process will help the designer to identify any lane inconsistencies and remedy them prior to actually preparing the design concept drawing. Closely Spaced Roundabouts When evaluating two more closely spaced (i.e., approximately 300 to 400 feet or less from each other) intersections with either one or both of the intersections being roundabouts, care should be taken to minimize lane changing on the common road link between the intersections. An analysis of this issue would typically entail the calculation of traffic volumes by lane on each of the entrances with traffic traced through the roundabout in order to calculate the volume in each exiting lane. The volume by lane exiting one intersection can then be compared to the anticipated traffic arriving by lane at the adjacent intersection. Sizable discrepancies between the volumes in any one lane would likely result in weaving between the two intersections. In some situations, this is not problematic. However, under heavy flow or saturated conditions, it is undesirable to rely upon lane changing in order for a particular scheme to succeed. If analysis shows that lane changing is likely, this situation can often be rectified by changing lane assignments or adding strategically located lanes. Driveways and Access Management Similar to other intersection types, it is desirable that driveways be located as far as practical away from the intersection, and drives should not be located along the entry and exit radii between the adjacent legs of a roundabout. While roundabouts typically do not experience traffic queuing like other intersection types, splitter islands may block left turns in and out of driveways near the intersection. At the same time, roundabouts can be used for U-turns which will usually enhance access. In certain circumstances, it is acceptable for a driveway to enter a roundabout as if it was an approach. These situations require careful analysis. The general principles outlined in MDOT s Access Management Guidebook (MDOT, 2001) should be followed when considering driveway access near a roundabout. However, every situation needs to be evaluated individually, and unique situations may justify deviating from the recommendations in the access management guidebook. 19

24 SECTION 4: GEOMETRIC DESIGN Pavement Markings and Geometry At multilane roundabouts, pavement markings are an integral part of the design concept layout and should be developed at the same time that the initial geometry is identified and drawn. If not, redesign can be required once pavement markings are considered later. Pavement markings are addressed in Section 5 of this document. Roundabouts With More Than Four Legs At intersections with more than four legs, realigning one or more low-volume legs out of the intersection (i.e., forming a new "T" intersection a short distance from the roundabout) or constructing a cul-de-sac on one or more legs should be considered as a first option (with the goal of creating a four-leg intersection). If this option is not feasible, the following items need to be taken under consideration. At roundabouts with more than four legs, there are some special considerations. First, it can be difficult to achieve speed control on the approaches without making the ICD larger than what would be needed with a four-leg intersection. Second, designers should try to maximize the angle between legs (this may require a larger ICD in some circumstances). Third, care should be taken in order to accommodate right turns by trucks as this can be difficult to accomplish when there is a small angle between legs (additional information regarding trucks at roundabouts is provided later in this section). Finally, it can be difficult to develop an acceptable pavement-marking scheme at multilane roundabouts with more than four legs. Curb Types Curb types at roundabouts should be as follows: Between the circulating road and the truck apron or between the circulating road and central island if no truck apron is present, type D curb (as specified in the MDOT Design Manual) should be used. For the splitter islands and the outside curbs, type B curb (as specified in the MDOT Design Manual) should typically be used. However, if the design speed is 50 mph or less and the posted speed is 45 mph or less, type F curb can be substituted for the splitter islands and outside curbs. Section 4.5: Three-Lane Roundabouts Three-lane roundabouts are not covered in the FHWA Guide and should be utilized only when two-lane roundabouts would not provide acceptable traffic operations. Because guidance regarding the design of three-lane roundabouts is not provided in the FHWA Guide, designers should proceed with caution and contact/work with the MDOT GDU. Most of the general principles outlined in the FHWA Guide regarding two-lane roundabouts would also apply to the design of three-lane roundabouts. The maximum R1 value for a three-lane roundabout should be 328 feet using a fastest vehicle path that is offset from a painted centerline and curb faces by 3 feet. These values are used for three-lane roundabout design in the United Kingdom where such roundabouts are common. 20

25 SECTION 4: GEOMETRIC DESIGN Figure 5: Typical Three-Lane Roundabout (Sterling Heights, Michigan) Section 4.6: Accommodating Trucks It is very important to consider the design vehicle to be accommodated, as large vehicle paths can greatly influence the geometry of a roundabout. Therefore, roundabouts must be designed to accommodate the largest vehicle likely to regularly use an intersection. Before the design process begins, the design vehicle should be identified. The types of roadways involved and the setting where the intersection is located will help determine the design vehicle. A WB-62 should be used as the design vehicle for intersections at freeway interchanges. On state trunkline intersections not located at interchanges, WB-50 should be used as the minimum design vehicle (WB-62 may be appropriate in some cases). MDOT staff should be consulted early in the planning process to identify the appropriate design vehicle for each specific intersection. Single-Lane Roundabouts As noted above, truck paths can greatly influence the geometry of a roundabout. This is especially true for single-lane roundabouts where accommodating trucks can actually influence geometry to a greater extent than the capacity analysis. Since single-lane roundabouts typically have smaller geometric features than multilane roundabouts, it is usually more difficult to accommodate larger trucks at single-lane roundabouts. In order to accommodate trucks, the entry and exit widths are typically 18 to 20 feet wide (curb face to curb face). While the circulating road is typically 1.0 to 1.2 times the width of the widest entry, it is not uncommon for single-lane roundabouts to have a circulating road width of 22 to 25 feet. Entry and exit width must be determined for each intersection on a case-by-case basis because the widths depend on a combination of factors including the design vehicle, angle of the approaches and exits, radius of entries and exits, location of the entries/exits relative to the ICD and each other, and the ICD of the roundabout. For additional design details see Section of the FHWA Guide. Sometimes aprons may be required to provide an additional traversable area around the central island for large trucks to travel through the roundabout. Left and U-turns should be used to determine the width of an apron. Vehicle turning path templates from AASHTO s A Policy on Geometric Design of Highways and Streets Guide (AASHTO, 2004) can be used to help in determining an initial width for the circulating 21

26 SECTION 4: GEOMETRIC DESIGN road and apron. However, CAD-based vehicle turning path software should also be used in all cases to determine the swept path of the design vehicle. This is an important step in the design process since the factors noted above can create a complex situation that cannot be addressed by simply applying the AASHTO turning templates. Turning path software should be used to check right turns as well since these can actually be the most difficult to achieve at a single-lane roundabout. Multilane Roundabouts Multilane roundabouts can be designed in two different ways to accommodate large trucks. The most commonly accepted way to design a multilane roundabout is to assume a truck will use two lanes to enter, circulate and exit the roundabout. Alternatively, a roundabout can be designed so that trucks can remain in one lane as they traverse the intersection. This approach is less commonly used since overall geometry must be larger, possibly resulting in increased ROW needs, higher cost, and a potential for increases in certain types of crashes. As with single-lane roundabouts, the left/u-turn determines the width of the truck apron. When determining the apron width, designers should assume a worst case scenario which is that a truck s cab/front tires will stay completely within the inside circulating lane. As with single-lane roundabouts, vehicle turning path templates can be used, and turning path software is highly recommended. Figure 6: Typical Single-Lane Roundabout (Gaylord, Michigan) 22

27 SECTION 4: GEOMETRIC DESIGN Section 4.7: Cross Slope The circulating road should have a cross slope of 1.5 percent sloping down away from the raised central island at freeway interchange intersections and on state trunklines. The truck apron should also have a cross slope of 1.5 percent away from the center of the central island. Additional details regarding vertical considerations at roundabouts are provided in Section of the FHWA Guide. Achieving proper drainage at a roundabout can be complicated, and considerable care should be taken when preparing detailed grades. In some situations, it can be advantageous to tilt the entire intersection in order to facilitate drainage. Regardless, great care should be taken in order to prevent flat areas and to ensure that positive drainage is achieved at all locations. Particular attention should be given to the transitions between the approaches and a circulating road. In some instances, it can be helpful to develop a profile for the circulating road. Where a roundabout is located near a horizontal curve with superelevation, adequate distance should be provided for the roadway to transition out of the superelevation before entering the roundabout. Section 4.8: Sight Distance As with any type of controlled intersection, stopping sight distance must be provided so that motorists can recognize the need to slow down and stop if necessary. Section of the FHWA Guide should be followed when calculating stopping sight distances for the approaches, in the circulating road, and to the crosswalks. Regarding intersection sight distance, Section of the FHWA Guide provides one method for calculating required visibility to the left as a vehicle approaches a roundabout. However, this section of the FHWA Guide recommends that approaching drivers be able to see a considerable distance up the preceding approach which can be problematic in some situations. An alternative method for calculating sight distance to the left (based on research from the United Kingdom) requires that drivers approaching a roundabout need only be able to see to the yield line of the entry to their left when they are approximately 50 feet before the yield line. Either of these two methods is acceptable for calculating sight distance to the left. In addition, research has shown that restricting visibility to the left until a driver is approximately 50 feet before the yield line on an approach will reduce crashes. Restricting vision in this way does not interfere with intersection stopping sight distance. There also may be benefits to making the central island more visible and reducing sight lines through the central island to the opposite side of the roundabout (see Section 6.3 for landscaping within central islands and approaches). Designers should consider restricting this visibility where practical. When considering the location of signs, designers should take care to ensure that these will not block any clear sight areas. The only exception to this general guideline would be for chevron signs in the central island which usually need to be located within a sight area. Last, designers should take into account the fact that there may be up to 12 inches of snow present on the central island at some times and account for this when calculating the sight lines. If the location of the roundabout is within or near a horizontal curve, careful analyses should be conducted to ensure adequate sight distance is provided. 23

28 SECTION 4: GEOMETRIC DESIGN Section 4.9: Pedestrians As noted in the FHWA Guide, the design of pedestrian facilities at roundabouts involves a balance among pedestrian convenience, pedestrian safety, and roundabout operations. Modern roundabouts accommodate pedestrians more safely than traffic signals due to the reduced number of conflicts points, lower speeds, shorter exposure times, and the presence of a median refuge which allows crossing one direction of traffic at a time (FHWA Guide Exhibits 5-5 and 5-6). At roundabouts, pedestrians are accommodated by crossings around the perimeter of the intersection. The roundabout should be designed to discourage pedestrians from crossing into the central island. As with any intersection, careful consideration is required when designing pedestrian facilities and must be considered during the planning phase. Pedestrian Facility Type The designer first must determine if pedestrian facilities are warranted based on the overall setting/location of the intersection, existing pedestrian volumes, and local government plans for future non-motorized facilities. While pedestrian facilities are typically warranted in urban settings, other locations (such as some rural intersections, interchanges and/or limited access roadways) may not require them. As with signalized intersections, all pedestrian facilities should meet ADA design standards. Next, the designer must determine if pedestrian facilities are desired, and what facility type is appropriate. Appropriate facility types can be identified based upon traffic and pedestrian volumes (see pages 19 and 20 of Roundabout Design Guidelines [Ourston, 2001] for one method of determining types). If there is any doubt about the best facility types, designers should err on the side of caution and include a zebrastyle crosswalk with a median refuge in the splitter island. In some unusual situations, pedestrianactuated crosswalk signals may be deemed necessary to safely accommodate large volumes of pedestrians. In this situation, a detailed analysis must be conducted to determine if the pedestrian signals would impact roundabout operations. Pedestrian underpasses have also been used where large volumes of pedestrians are present. Design Considerations The following factors should be considered when designing pedestrian facilities at a roundabout. Crosswalks should typically be located one car length before the yield line. The location and orientation of crosswalks need to be considered in order to minimize the pedestrian crossing distance. Each pedestrian crossing also needs to be designed to provide maximum visibility between pedestrians and vehicles. Pedestrian crossings should be located to use the splitter islands as a refuge. For splitter island design (including minimum widths) see Exhibit 6-26 of the FHWA Guide. The pedestrian crossing should also be close enough to an intersection to prevent pedestrians from crossing at a less safe location and to minimize out-of-direction travel. Bypass Lanes Free flow, right-turn bypass lanes should typically be avoided in areas with high pedestrian volumes. However, there may be specific limited circumstances where such designs cannot be avoided. In these cases, it is recommended that the designer consider whether alternate designs are possible, including the possibility of a semi right turn bypass lane where vehicles in the bypass lane yield to exiting traffic. Visually Impaired Pedestrians Nationally, some visually impaired pedestrians have expressed concern about their ability to safely cross at modern roundabout intersections without pedestrian actuated signalized crosswalks. This issue and 24

29 SECTION 4: GEOMETRIC DESIGN potential accommodations are presently being studied, but there is no clear guidance available at this time (the Access Board has prepared draft guidelines which are currently being debated). Until such time as clear direction is available, designers should consider the potential effects to visually impaired pedestrians where they are known to be present. Section 4.10: Bicycles Studies have shown that bicyclists are more safely accommodated on multi-use paths around the perimeter of a roundabout than within the circulating roadway. This issue is more of a concern with multilane roundabouts than single-lane roundabouts. If on-street bicycle facilities are provided at an intersection where a roundabout is being designed, accommodations should be included to direct approaching bicycles off the roadway and onto a multi-use path (see Exhibit 6-38 in the FHWA Guide). Bicycle lanes should never be provided within the circulatory lanes of a roundabout. In situations where no bicycle facilities are present and bicycle numbers are relatively low, special facilities need not be provided. 25

30 SECTION 5: PAVEMENT MARKINGS AND SIGNING Section 5.1: Introduction Pavement markings are an important part of any roundabout design and should be an integral component starting with the early stages of concept design. Together with signing, they provide guidance to motorists as they approach and traverse a roundabout. This section of the document is intended to provide some basic principles regarding pavement markings and signing at roundabouts. Like the other sections of the document, it references the FHWA Guide and other sources for those readers interested in more details. This document is also intended to encourage uniformity in pavement markings and signings where this is practical. It is MDOT s intention to develop a more detailed chapter regarding pavement markings and signing in the future. This may include typical pavement marking sheets in the MDOT Design Manual. Section 5.2: Pavement Markings Some standard pavement markings are discussed in Section 7.2 of the FHWA Guide, and this section of the document can be followed for basic guidance. For additional pavement marking details see the Michigan Manual on Uniform Traffic Control Devices. At single-lane roundabouts, pavement markings are generally straightforward and are covered by the FHWA Guide. The FHWA Guide does not address pavement markings at multilane roundabouts, so some basic principles are provided below. Figures 7 and 8 show some of the basic terminology that is typically used for pavement markings at multilane roundabouts. Pavement Marking Principles The main objective of using pavement markings at multilane roundabouts is to provide direction to motorists so that they can traverse a roundabout without changing lanes. Using pavement markings at multilane roundabouts will typically improve safety and traffic operations and educate drivers about lane use. At some intersections, they are critical to the successful functioning of a roundabout. This is especially true where unusual peak hour turning patterns (i.e., double right turn, double left turn) occur. Pavement markings can also be used to retrofit older roundabouts or traffic circles with problematic geometry. Due to the complexity involved with pavement markings at multilane roundabouts, designers should proceed with caution. 26

31 SECTION 5: PAVEMENT MARKINGS AND SIGNING Figure 7: Roundabout Pavement Marking Terminology Figure 8: Roundabout Pavement Marking Terminology (Continued) 27

32 SECTION 5: PAVEMENT MARKINGS AND SIGNING Basic pavement marking principles at multilane roundabouts include: 1. Turning patterns/volumes must be accommodated without inconsistencies (i.e., the marking scheme must accommodate all of the individual movements without requiring lane changing inside the roundabout) during the same peak hour and during different peak hours (i.e., AM versus PM peak hour). This can be accomplished by tracking each movement through the intersection for both peak hours. If conflicts within the same peak hour or between different peak hours cannot be resolved after trying different schemes, two options remain. Sometimes, partial spiral markings that accommodate the major traffic streams can be utilized as long as they do not create conflicts. The second option would be to not use markings in the circulatory roadway. 2. Lane use control arrows should designate and reinforce correct lane usage. 3. Lane changing within the circulating roadway should be prevented. This is achieved by getting motorists into the correct lane before they reach the yield line and having lane consistency throughout the entire intersection. Having relatively long approach lanes helps this objective because motorists have more time to get into the correct lane before the yield line. In this situation, lane use control arrows can be repeated on the approaches with two or three sets of arrows being desirable. 4. Approach arrows should define the exit road that is accessed from each specific approach lane and should be consistent with lane use signs. 5. Approach arrows should be oriented relative to exit roads. 6. Approach arrows/stripes must be consistent with circulating arrows/stripes and signs. 7. Unbalanced lane use (i.e., most vehicles using one lane vs. balancing out evenly on two or three lanes) should be prevented by the selection of proper lane designations which achieve the most even distribution of traffic possible. 8. On flared approaches (i.e., adding one or more lanes on an approach), lane stripes should extend back from the yield line as far as reasonably possible. 9. Line types must send the correct message. In general, it is recommended to use a 2:1 ratio of mark to gap with a 12-foot mark and 6-foot gap being preferred for the lane lines on the approaches, circulating road, and exits. 10. Pavement markings and signs need to be an integral part of the geometric design of a roundabout and should be developed concurrently with a concept design. 11. Concentric (i.e., bull s eye ) circles must never be used in the circulating road. These markings cause a number of problems including indecision, lane use imbalance, decreased capacity, and the potential for exit crashes. 12. Markings at closely spaced roundabouts should function as one integrated system. The objective here is for drivers to select the lane they need for their ultimate destination before entering the system and to traverse multiple intersections without changing lanes. In some cases, extra lanes that are not needed solely for capacity purposes may need to be added for lane continuity. 13. Guide dots should typically be used to direct motorists from the yield line into the proper circulating lane. It is recommended that a 6 wide broken white line be used for this mark with a 1-foot mark and a 10 gap. 28

33 SECTION 5: PAVEMENT MARKINGS AND SIGNING Figure 9: Typical Two-Lane Roundabout (South Bend, Indiana) Left-Turn Lane Use Arrows There has been some debate in the past as to whether the left turn lane use arrow should be used on roundabout approaches. Those questioning this practice have been concerned that drivers could mistakenly turn left at the yield line and travel the wrong way on the circulating road. While this potential risk should be considered, not using the left turn arrow on approaches has clearly and repeatedly resulted in lane changing/weaving at roundabout exits, resulting in crashes. In addition, drivers receive clear and repeated direction from pavement markings and signing that they should circulate in a counterclockwise direction in the circulating road. Therefore, the left turn lane use arrow should be used as appropriate on approaches where left turns will be conducted. NCUTCD Guidelines At the present time, the National Committee on Uniform Traffic Control Devices (NCUTCD) has approved draft guidelines regarding pavement markings at roundabouts. While this information has not yet been formally approved for use in the United States, it can be followed as long as it does not conflict with the principles defined above or the FHWA Guide since the alternative is to have no guidance regarding multilane roundabouts. Note that the circulating road markings (a combination of a solid line and a short dashed line) in the draft NCUTCD should not be used. Instead, a line with a 12-foot mark and 6-foot gap should be used for the circulating road. Spiral Hatching In situations where one lane in the circulating road needs to be spiraled out away from the central island, a spiral hatching near the central island can be used. Anecdotal reports from around the United States indicate that in some situations, drivers tend to ignore these markings, drive over them, and do not transition into the desired lane. As an alternative to using spiral hatching, it is acceptable for designers to use an irregular shaped central island with the curbed island at the same location where the spiral hatching 29

34 SECTION 5: PAVEMENT MARKINGS AND SIGNING would have otherwise been used. If an irregular shaped central island is used, designers need to ensure that trucks can still fit through the circulating road after changing the shape of the central island. Curb Faces Curb faces can be painted with reflective paint as an optional treatment. In some instances, this is used to aid drivers in identifying curb locations at night. Typically, waterborne paint is used for this application with the intent that it will be a temporary measure which is not replaced once it wears off since drivers will presumably be familiar with the intersection by that time. Section 5.3: Signing Typical signing at a roundabout, as described in Section 7.1 of the FHWA Guide, should be followed for most situations (see Figure 10). In order to enhance safety and traffic operations, lane use control signs should be used on all approaches at multilane roundabouts in order to give motorists advance notice as to which lane to use for each movement. Lane use control signs shall be consistent with the lane use arrows painted on the pavement. Designers should check the signs in Figure 10 against the most recent version of the MUTCD to ensure that the current approved signs are used. Figure 10: Typical Roundabout Signing (Source: FHWA Guide. For text reference related to pedestrian warning sign, see section of the FHWA Guide) 30

35 SECTION 5: PAVEMENT MARKINGS AND SIGNING Yield Signs At multilane roundabouts, yield signs should be used on both the splitter island and the outside. On the splitter island, the yield sign should be oriented so that it is perpendicular to traffic as it approaches the roundabout approximately 300 to 400 feet prior to the yield line. This yield sign should be located at the corner of the splitter island where it intersects with the yield line. The yield sign on the outside should be oriented so that it is perpendicular to traffic that has already negotiated the entry curvature and is near or at the yield line. This sign should be located perpendicular to the location where the yield line meets the lane demarcation line for the outside lane. Multiple yield signs can also be used for single-lane roundabouts if preferred. If desired, designers can use extra large (36-inch) yield signs to improve their visibility. Chevron Signs Regarding chevron signs in the central island, designers have the choice to use a single chevron plate or a series of single chevrons that are placed in a staggered pattern in order to maximize visibility for drivers on the circulating road. Chevron signs should be located in the central island so that they are perpendicular to and directly in front of approaching vehicles when they are approximately 300 to 400 feet before the yield line. Chevron signs should not be located within the truck apron, but should be located just inside the apron in the central island. Illuminated Bollards Illuminated bollards (Figure 11) are often used at roundabouts in the United Kingdom and appear to provide safety benefits. Their use in the United States has thus far been limited as they are not approved by the Manual on Uniform Traffic Control Devices (MUTCD) as a traffic control device in the form shown in Figure 11. Bollards are typically installed at the nose of the splitter island (i.e., the end of the island farthest away from the circulating road), aiding drivers during periods of low visibility. At this time, the use of illuminated bollards which display a traffic control device would be considered experimental and require the necessary approvals on a case-by-case basis. An illuminated bollard without any traffic control devices displayed on it would be permitted. Figure 11: Illuminated Bollards 31

36 SECTION 5: PAVEMENT MARKINGS AND SIGNING Destination and Lane Use Signs Designers have the option of using advanced destination guide signs. Often these utilize a diagram (see Section 7 of the FHWA Guide) that is intended to help motorists identify their destination and which leg of the roundabout they will exit in order to get there. At multilane roundabouts, motorists must make the mental connection between the diagram sign and the subsequent lane use regulatory sign and decide which lane will bring them to their intended destination. Two options are available to designers who desire to combine this information into one sign. These options make it easier for motorists to select a lane based on their destination and also reduce the number of signs that need to be mentally processed by a motorist at a multilane roundabout. Figure 12 shows an example of a guide sign that includes lane assignments and destinations on the same sign. Note that this sign uses standard lane assignment arrows with a circle to indicate the left turn occurs after the central island. Figure 12: Combined Destination and Lane Use Guide Sign A second option available to designers is to add destinations on to a lane use regulatory sign. Figure 13 shows an example of this. Note that Figure 13 uses the fishhook style of lane arrows. Figure 13: Combined Destination and Lane Use Regulatory Sign 32

37 SECTION 5: PAVEMENT MARKINGS AND SIGNING Similar to the left turn lane use arrow painted on the pavement, the left turn lane use sign should also be implemented on all approaches where such a movement is possible. The benefits of using this type of sign outweigh any potential risks. Miscellaneous On high-speed approaches, it is acceptable for designers to use flashing beacons mounted on the roundabout ahead warning sign or other appropriate locations as identified on a case-by-case basis. Designers should take care to avoid excessive signage since this has the potential to confuse or overwhelm motorists. In cases where pedestrian volumes are very low, pedestrian warning signs can be eliminated to reduce the number of signs. Sign heights must not block views of pedestrians. 33

38 SECTION 6: OTHER DESIGN AND OPERATIONAL CONSIDERATIONS Section 6.1: Introduction This section contains information regarding other topics not addressed in the preceding sections. Section 6.2: Lighting In accordance with State law, MDOT may not pay electric bills in non-freeway settings. Detailed discussions regarding which agency will maintain and pay for electricity should occur with local government representatives and utility companies before a final decision is made regarding roundabout lighting. At night, drivers must be able to distinguish the layout and operations of a roundabout prior to entering, while circulating, and as they exit. Proper lighting assists drivers as they complete these maneuvers. Therefore, lighting should be installed at all roundabouts unless unusual circumstances preclude installation. The need for lighting is determined based on the setting where each roundabout is located. In urban settings, illumination should be provided as typically all approaches are illuminated. For suburban settings, lighting is recommended when one or more of the approaches are illuminated, when an illuminated area in the vicinity can distract a driver, or when heavy nighttime traffic is anticipated. In rural situations lighting is recommended unless establishing a power supply would be prohibitively expensive. If no lighting is provided, the roundabout should be well signed, and reflective markings should be used. In addition, if lighting is not provided, advanced warnings on the approaches should be emphasized and could include solar-powered flashers on roundabout ahead warning signs or more/larger chevrons in the central island. Lighting plans should be customized to the specific geometry of the roundabout, site constraints, photometric requirements, and equipment options. It is preferred to light from the outside in towards the center by arranging the light poles around the perimeter of the roundabout. In order to illuminate the approaches, lighting should be provided 400 feet upstream of the entry. Light poles should not be placed within the central island or splitter islands unless there is no other practical option. It is also desirable to keep poles out of the clear zones. If light poles are to be located in clear zones, frangible base poles should be used per the recommendations of the current AASHTO roadside design manual. See the FHWA Guide Exhibit 7-23 for recommended illumination levels. Illuminated bollards (see Section 5 of this document for more information) are another option for lighting as they increase visibility of a roundabout at night. These are located on the splitter island at the end farthest from the roundabout. Section 6.3: Landscaping Roundabouts provide numerous landscaping opportunities that are consistent with a context sensitive solutions philosophy. In addition, landscaping can provide safety benefits. Landscaping within the central island makes it more visible to approaching traffic and encourages speed reduction on approaches. Plantings along the approaches and splitter islands can also channelize pedestrians to desired crossing locations and discourages them from crossing to the central island. Landscaping of the central island 34

39 SECTION 6: OTHER DESIGN AND OPERATIONAL CONSIDERATIONS should not encourage pedestrian traffic and should avoid the use of street furniture. Landscaping materials should meet the recommendations of the 2002 AASHTO roadside design manual. Sight distance requirements are discussed in Section 4 of this document. The location and type of plant materials selected should not interfere with required sight areas. Low-growing plants with a maximum height of 18 inches should be planted within restricted sight areas. Large plants, bushes, and trees may be planted outside of the required sight areas. If these large plants, bushes, and trees are not included in a landscaping plan, it is recommended that a species of tall grass be planted in the central island outside clear sight areas. The tall grass will help block headlights at night, make the central island more conspicuous, and reduce maintenance. Large fixed objects should be avoided in areas where vehicle runoffs may occur. A realistic maintenance plan should be considered when designing any landscape plan. A formal maintenance agreement with the applicable local government agency or a reputable community group is required for high maintenance landscapes. Liability issues should be addressed as a part of such an agreement. If there is no local interest in maintaining a landscape, plant materials that require little to no maintenance, such as tall grass, should be used. Any landscaping plan should be consistent with the setting and should be reviewed by one of MDOT s landscape architects. Hearty plants that require minimal maintenance, are salt tolerant, and are appropriate for the local climate should be used. As a general rule, splitter islands should not contain trees or planters, and trees with large canopies should also be avoided. Section 6.4: Maintenance Snow plowing of roundabouts is successfully performed across the country in areas with very high annual snowfall amounts. Roundabouts are typically plowed from the inside (i.e., starting on the truck apron or at the inside edge of the central island) toward the outside with either individual or multiple plows. Entries and exits are then plowed by pushing the snow to the outside curb. Section 6.5: Ongoing Monitoring Traffic operations (e.g., counts and observed queues/delays) and safety (e.g., crash data) at newly constructed roundabouts should be evaluated at three-, six-, and 12-month intervals after construction. After that, they should be monitored at least annually. Assuming that geometry, pavement markings, and signing are constructed according to plans, monitoring can help identify whether minor adjustments to these elements may be advisable. Collection of this data also allows for comparison of the roundabout to the previous intersection performance. It is also important to remember that there will be an initial learning period during which time crash frequency and rates may be higher than the long-term average, and this should be taken into consideration. An example monitoring report is included in Appendix C. The use of internet web cameras is also encouraged at roundabouts where the cost is reasonable, particularly in urban settings. Section 6.6: Pavement Type Both asphalt and concrete pavement are acceptable for use at roundabouts. Each pavement type has advantages and disadvantages that need to be evaluated on a case-by-case basis at each intersection. If asphalt is selected, an appropriate pavement design must be used. If concrete pavement is used at multilane roundabouts, a joint layout pattern should be developed that matches the pavement-marking scheme with particular attention paid to the circulating road and exits. In addition, when concrete is used, 35

40 SECTION 6: OTHER DESIGN AND OPERATIONAL CONSIDERATIONS designers should consider the use of black borders around the edges of white pavement markings in order to make the markings more visible. Section 6.7: Utilities When designing a roundabout, the location and type of utilities surrounding the intersection must also be taken into account on a case-by-case basis. Some utility components may be located in splitter and central islands, but above-ground features must remain outside of all clear zones and not cause a potential safety hazard for vehicles. Future maintenance of utilities should also be taken into account when designing a roundabout. Section 6.8: Maintenance of Traffic Maintenance of traffic can be accomplished in different ways depending on case-specific circumstances. In some instances, part width construction may be appropriate, while in others, it is possible to close the intersection and detour traffic. Within the part width construction category, there are many possible ways to stage construction of a roundabout depending upon how the roundabout is situated relative to the existing roadways. If part width construction is used, traffic should be routed through the roundabout in a counterclockwise direction whenever possible in order to train drivers as to the proper direction of travel at the intersection. This is especially important during the final stages of construction prior to opening the intersection. 36

41 SECTION 7: PUBLIC INVOLVEMENT Section 7.1: Introduction The public involvement process is very important when a roundabout is under consideration. Although it is not always possible to achieve, the goal regarding public involvement should be building consensus and support for the road improvements under consideration. This section of the document provides basic information about educating the public and obtaining public input regarding roundabouts. Appendix A contains example materials that can be used in the public involvement process. Section 7.2: Educating the Public Public acceptance of roundabouts can be one of the biggest challenges faced when a roundabout appears to be a good technical solution. Although roundabouts are becoming more common in Michigan and throughout the United States, misconceptions still exist due to unfamiliarity and failure to distinguish roundabouts from old style traffic circles. Therefore, public involvement and roundabout education are very important first steps in leading toward acceptance of a roundabout. The experience in Michigan has been similar to other states with public resistance being common prior to construction of a roundabout. Once the roundabout has been constructed and is operating, public opinion has been predominantly favorable at most intersections. Due to misunderstandings about roundabouts among the public, public involvement campaigns should include roundabout education for elected officials, staff members, and the general public. These types of public involvement processes allow local governments to partner with MDOT. The following topics should be considered when educating the public about roundabouts: Basic roundabout concepts/terminology. Modern roundabouts versus traffic circles Due to public perception/experience with traffic circles it is very important to differentiate between the two (refer to Appendix A for the differences between modern roundabouts and traffic circles). Safety benefits Provide data showing the increased safety benefits of roundabouts when compared to other intersection types. Locations around the state Roundabouts have been accepted in other Michigan communities. Use at high speed intersections. Cost/ROW impacts compared to other options. Older and inexperienced drivers at roundabouts. How to drive a roundabout Allows public to have basic understanding of how roundabouts operate. Traffic operations Improvements compared to other intersection types. Trucks Snow removal Driveways near roundabout Pedestrians at roundabouts Shows pedestrians can be safely accommodated at roundabouts. Bicycles 37

42 SECTION 7: PUBLIC INVOLVEMENT Section 7.3: Public Involvement Techniques Public involvement techniques for roundabouts are similar to what would be utilized for other road projects. The timing of public input regarding a roundabout is important. Input should be sought after initial investigations have been conducted and a roundabout has been determined feasible by MDOT, but before design commences. Before design, consideration of public input regarding roundabouts should take into account the degree to which public education has occurred. Public involvement processes should be adapted to each individual project based on coordination with local officials, the MDOT project manager, and the MDOT public relations staff. Public information meetings are an excellent forum that can be used to educate the public about roundabouts and to hear their feedback. Not only do public meetings give the opportunity to dispel roundabout myths, they allow the public to be involved in the planning process and have their questions and concerns addressed. Informational materials at public meetings could include the following: Exhibits mounted on foam core boards Brochures Videos before/after Pictures before/after Project-specific materials Public comment forms Websites are another excellent way to provide the public with roundabout information. There are many sites available that provide educational information. Example public involvement materials are provided in Appendix A. 38

43 SECTION 8: REFERENCES AASHTO A Policy on Geometric Design of Highways and Streets Guide, 5 th Washington, D.C. Edition. Insurance Institute for Highway Safety (IIHS) Crash Reductions Following Installation of Roundabouts in the United States. Arlington, Virginia. Lalani, N The Impact on Accidents of the Introduction of Mini, Small, and Large Roundabouts at Major/Minor Priority Junctions. Traffic Engineering and Control. London. MDOT The Access Management Guidebook (Reducing Traffic Congestion and Improving Traffic Safety in Michigan Communities). Lansing, Michigan. Ourston Roundabout Engineering Roundabout Design Guidelines. Santa Barbara, California. Rodel User s Manual, This software is based on the British regression equations and is available from Rodel Software Limited. Contact Barry Crown at rslcrown@aol.com or A windows-based Version 2.0 is currently under development. SEMCOG Traffic Safety Manual (2 nd Edition). Detroit, Michigan. TRL Accidents at 4-Arm Roundabouts. TRL Laboratory Report Crowthorne, Berkshire, United Kingdom. TRL The Traffic Capacity of Roundabouts. TRL Laboratory Report 942. Crowthorne, Berkshire, United Kingdom. United States Department of Transportation, Federal Highway Administration. Roundabouts: An Informational Guide. June Publication Number FHWA-RD

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45 The factual truths about roundabouts... When considering the modern roundabout as an intersection alternative, a number of common misconceptions are too often presumed by members of the public, elected officials, consultants and even transportation experts who are unfamiliar with this type of intersection control. Some factual truths concerning the modern roundabout are: 1. The modern roundabout is significantly different than old-style traffic circles or rotaries. 2. When designed properly, modern roundabouts are safer than traffic circles and traditional signalized intersections, and are often used as a calming effect to slow traffic. 3. Roundabouts increase road capacity as they can handle high traffic volumes. They may also require fewer lanes or reduced median widths as they can reduce congestion and backups. 4. Roundabouts are an effective treatment for rural intersections where signalization may not be appropriate. 5. Roundabouts can provide adequate downstream gaps for motorists entering the roadway from side streets or out of driveways. 6. Roundabouts are inexpensive to operate, easily modified and are low maintenance. 7. Roundabouts safely accommodate high volumes of pedestrians and bicycles. 8. Roundabouts reduce speed and cause less backups than stop and go traffic. 9. Roundabouts can be landscaped to be aesthetically attractive. History of the Modern Roundabout The modern roundabout was developed by British engineers in the 1960s and 1970s. During that time, engineers analyzed traffic volumes and crash records from hundreds of intersections and experimental layouts. Based on this research, they deduced the precise intersection shape and characteristics that would carry vehicles most efficiently and safely. Since 1987, the British have had the lowest highway fatality rate of any country in the world (20% lower than the United States). Roundabouts are heavily used in Britain, France, Australia, Switzerland and many other countries. Modern roundabouts have been constructed at hundreds of intersections throughout the United States over the past ten years.

46 elatively new to the United States, modern roundabouts have been common throughout other portions of the world for several decades as an alternative to stop-controlled and signalized intersections. The main characteristic of a modern roundabout as the "yield-at-entry" rule, meaning that traffic entering a roundabout must yield to the traffic already within the roundabout. Additionally, studies show a roundabout to be one of the safest types of intersections available compared to a signalized intersection. 40% reduction in total crashes; 80% reduction in injury crashes; and 90% reduction in less serious injury/fatality crashes. Studies also indicate that pedestrians are 50% less likely to be hit at a roundabout than at a signalized crosswalk. The vehicular crashes that do occur at roundabouts tend to be lowspeed sideswipes or rear-end collisions versus head-on, left-turn and high-speed broadside collisions that are more frequently experienced at signalized intersections. How does a roundabout differ from traditional intersection signalization? Roundabouts have efficient traffic operations by keeping vehicles moving. This results in less delay, fuel consumption and pollution. In addition, roundabouts save money used to maintain signal equipment and provide electricity. Another noticeable difference between a traditional signalized intersection and a roundabout are the safety factors. Traffic signals tend to encourage drivers to accelerate their vehicles through intersections in order to "beat the red light" and thus; speeds are generally higher (35 mph avg.), which can ultimately increase the number of crashes, injuries and fatalities. The comparison charts (right) show the difference in conflict points that motorists can encounter at a typical signalized intersection versus at a roundabout. How do roundabouts differ from traffic circles? Although "traffic circles" and modern roundabouts utilize round circulating roadways, there are major differences in the design and operation of these types of intersections. A modern single-lane roundabout is typically smaller in diameter than the traffic circle. This can result in lower speeds and safer conditions. Single-lane roundabouts are generally 100 to 150 feet in the outer diameter as opposed to the two older-style circles, which are typically 150 to 300 feet diameter or larger. Because traffic circles are larger, they move more vehicles at higher speeds ranging from 30 to 45 mph. Roundabouts are smaller and thus, force vehicles to slow to speeds ranging to 15 to 20. This difference is significant for several reasons: a. Vehicles enter roundabouts more easily due to the lower speeds (15-20 mph) on the circulating roadway. b. A traffic circle permits higher speeds making entering the circle more difficult, which can result in more frequent and severe traffic crashes. c. Higher speeds on traffic circles make crossings for pedestrians more dangerous. d. The flared approach and proper entry angle on a roundabout allows vehicles to safely yield on all entries when merging into the circulating traffic. This allows for shorter delays as drivers easily merge into the circle by slowing for circulating traffic, not stopping. e. Properly planned roundabouts are designed using rigorous standards based on specific traffic turning volumes. On the other hand, the size of traffic circles is based on land availability and distances needed to accommodate high speed weave movements. f. Roundabouts can accommodate large vehicles, including fire trucks and other emergency vehicles. g. Roundabouts are typically smaller than traffic circles, which results in cost savings for construction and maintenance.

47 Choose correct lane before you reach the yield line All traffic must yield here before entering Choose correct lane before you reach the yield line Exit the roundabout Exit the roundabout All traffic must yield here before entering Important Notes Select the correct lane before you reach the yield line. You must yield to traffic in the roundabout before entering. 81 Signs and pavement markings will help guide you. Choose correct lane before you reach the yield line Trafic in bypass lane does not yield Merge with other lane here Allow adequate space for large trucks. Right turn bypass lanes do not yield.

48 Stay in your lane as you circulate and exit. Do not change lanes! Stay in the same lane as you enter (i.e., left lane at the yield line uses left lane in roundabout) Stay in the same lane as you enter (i.e., left lane at the yield line uses left lane in roundabout) Choose correct lane before you reach the yield line All traffic must yield here before entering Stay in your lane as you circulate and exit. Do not change lanes! All traffic must yield here before entering Choose correct lane before you reach the yield line 1 2 VAN DYKE AVENUE Stay in your lane as you circulate and exit. Do not change lanes! All traffic must yield here before entering Choose correct lane before you reach the yield line From Southbound M-53 Important Notes Select the correct lane before you reach the yield line. Yield to pedestrians in the crosswalk. You must yield to traffic in the roundabout before entering. Stay in your lane as you circulate around the central island and exit. Signs and pavement markings will help guide you. Allow adequate space for large trucks. Left turns must be made from left entry lane (see above).

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50 1.0 Introduction This quick guide is a very brief summary that describes some basic roundabout terminology, identifies some of the situations where roundabouts could be used, outlines procedures for determining the feasibility of a roundabout, and summarizes how to perform operational analyses for roundabouts. Additional information and details regarding MDOT s policy can be found in MDOT s Roundabout Guidance Document. 2.0 Basic Terminology And Information The following terms are some of the most commonly used relative to roundabouts. A single-lane roundabout is a roundabout with one entering lane per approach. Two-lane roundabouts have at least one entry with two lanes separated by pavement markings and are more complex. Three-lane roundabouts have at least one entry with three lanes. A bypass lane or right turn bypass lane is typically used to accommodate heavy right turn movements to improve the capacity of a roundabout. The bypass lane is typically separated from an entrance by a curbed island and allows right-turning traffic to avoid entering the roundabout. The decision to use bypass lanes should take into account pedestrian and ROW constraints. In some cases, bypass lanes provide significant benefits, especially for rural interchanges. Entry Width is the width of an approach where it enters the roundabout. Flare Length is the distance over which the approach roadway widens to the entry width. Longer flare lengths give motorists more time to adjust and utilize all lanes. The Entry Radius is the radius of the outside curb at the entry. Inscribed Circle Diameter is also abbreviated as ICD. This is the outside diameter of the roundabout. The figure below also shows these common terms as they relate to an actual roundabout. MDOT Roundabout Quick Guide 1

51 Some other common elements are: Signing and pavement markings provides notification and guidance to drivers Lighting helps drivers identify intersection features such as splitter islands and curbs at decision points Landscaping can help to direct drivers attention where you want them to look and enhance aesthetics. Table 1 describes typical roundabout types and sizes based on traffic volumes. Table 1. Typical Roundabout Capacities and Inscribed Circle Diameters Type of Roundabout Approximate Peak Hour Capacity (Combined entering volume for all approaches) Inscribed Circle Diameter Compact Urban Up to 4,000 vehicles per hour Single Lane Up to 2,000 vehicles per hour Two Lane Up to 4,000 vehicles per hour Three Lane Up to 7,000 vehicles per hour Planning 3.1 Typical Locations and Applications Implementation of roundabouts can be beneficial to the traveling public in a wide variety of situations. The list which follows below identifies some of the most common locations and/or applications where installation of a roundabout may be advantageous. MDOT Roundabout Quick Guide 2