Roadway Vertical Alignments

Transcription

1 Roadway Vertical Alignments by Gregory J. Taylor, P.E.

2 INTRODUCTION This course summarizes and highlights the design of vertical alignments for modern roads and highways. The contents of this document are intended to serve as guidance and not as an absolute standard or rule. When you complete this course, you should be familiar with the general design of vertical roadway alignments. The course objective is to give engineers and designers an in-depth look at the principles to be considered when selecting and designing roads. Subjects covered include: Sight Distance (stopping, decision, passing, intersection) Vertical Alignment (grades, climbing lanes, passing opportunities, vertical curves) Coordination of Horizontal & Vertical Curves For this course, Chapter 3 (Section 3.4 Vertical Alignment) of A Policy on Geometric Design of Highways and Streets (also known as the Green Book ) published by the American Association of State Highway and Transportation Officials (AASHTO) will be used exclusively for fundamental geometric design principles. This text is considered to be the primary guidance for U.S. roadway geometric design. Copyright 2013 Gregory J. Taylor, P.E. Page 2 of 32

3 Background Geometric design is the assembly of the fundamental three-dimensional features of the highway that are related to its operational quality and safety by providing a smoothflowing, crash-free facility. Geometric roadway design consists of three main parts: 1) vertical alignment (grades and vertical curves) 2) horizontal alignment (tangents and curves) 3) cross section (lanes and shoulders, curbs, medians, roadside slopes and ditches, sidewalks) Together, these elements provide a three-dimensional layout for a roadway. In today s world, applying design standards and criteria to solve a problem is not enough. Designers must understand how all of the roadway elements contribute to its overall safety and operation. The fundamental objective of good geometric design will remain as it has always been to produce a roadway that is safe, efficient, reasonably economic and sensitive to conflicting concerns. Copyright 2013 Gregory J. Taylor, P.E. Page 3 of 32

4 SIGHT DISTANCE Sight distance is a major design control for vertical alignments and is defined as the length of roadway ahead where an object of specified height is continuously visible to a driver. This distance is dependent on the driver s eye height, the specified object height, and the height/position of sight obstructions. Sight distance is essential for the safe and efficient operation of vehicles. The three-dimensional features of the highway should provide a minimum sight line for these operations. Sight Distance Criteria Height of Driver s Eye: Height of Object: 3.50 feet above road surface (passenger vehicles) 7.60 feet above road surface (trucks) 2.00 feet above road surface (stopping & decision) 3.50 feet above road surface (passing & intersection) Stopping Sight Distance Stopping sight distance is considered to be the most basic form of sight distance. This distance is the length of roadway needed for a vehicle traveling at design speed to stop before colliding with an object in its path. Ideally, all of the roadway should provide stopping sight distance consistent with its design speed. However, this distance can be affected by both horizontal and vertical geometric features. Stopping sight distance consists of two distances: (1) Brake Reaction Distance Length of roadway travelled by the vehicle from driver perception to brake application Perception Braking A brake reaction time of 2.5 seconds is recommended for design and exceeds the 90 th percentile for all drivers. Research has shown this criterion to be inadequate for most complex driving conditions. Copyright 2013 Gregory J. Taylor, P.E. Page 4 of 32

5 (2) Braking Distance Roadway distance required to stop the vehicle (from the instant of brake application) after seeing an object on the road Braking Stopping It may be calculated from the following equation: design speed (mph) A recommended rate for deceleration is 11.2 ft/s². deceleration rate (ft/sec²) If the roadway is on a grade, this distance can be determined by: design speed (mph) deceleration rate (ft/sec²) grade (ft/ft) Copyright 2013 Gregory J. Taylor, P.E. Page 5 of 32

6 Stopping sight distance is a function of initial speed, braking friction, perception/reaction time, and roadway grade. It also contains assumptions about the driver s eye height (3.5 feet) and the size of object in the road (2 feet). This distance can be determined from the following formula: Stopping Sight Distance - Level Roadways design speed (mph) deceleration rate (ft/sec²) brake reaction time (2.5 seconds) Design Brake Reaction Braking Stopping Sight Distance Speed Distance Distance Calculated Design (mph) (ft) (ft) (ft) (ft) Source: AASHTO Green Book Table 3-1 Limitations of the AASHTO model Not fully accounting for heavy vehicles (longer to stopping times/distances) Not differentiating for various highway types Not recognizing differing conditions along the same highway. Copyright 2013 Gregory J. Taylor, P.E. Page 6 of 32

7 Good design should consider these variables by providing more than minimum stopping sight distance at locations with vehicle conflicts or hazardous conditions (sharp curves, cross-section changes, intersections, etc.). Decision Sight Distance Design sight distance recognizes that unexpected conflicts, driver decisions, or changes in the roadway can require additional time, and longer distances for the motorist to make a decision and safely navigate the vehicle. This sight distance provides greater visibility for drivers. This sight distance is the length of roadway required to detect a potentially hazardous situation, select an appropriate reaction, and perform the resulting maneuver. It also provides additional margin for error for a vehicle to maneuver safely. Decision sight distance should be provided for cluttered roadway environments, interchanges, intersections, lane drops, congested intersections, alignment changes, median crossovers, bridges, changes in roadway cross-section, toll collection facilities, and unusual geometric configurations. Decision sight values depend on the roadway s location (rural or urban) and the type of avoidance maneuver. Avoidance Maneuver Condition Time (sec) A Stop on rural road 3.0 B Stop on urban road 9.1 C Change on rural road 10.2 to 11.2 D Change on suburban road 12.1 to 12.9 E Change on urban road 14.0 to Copyright 2013 Gregory J. Taylor, P.E. Page 7 of 32

8 Decision Sight Distance Design Decision Sight Distance (ft) Speed Avoidance Maneuver (mph) A B C D E Source: AASHTO Green Book Table 3-3 Decision sight distances for Avoidance Maneuvers A and B can be calculated using the following formula: design speed (mph) driver deceleration rate (ft/sec²) pre-maneuver time (seconds) Decision sight distances for Avoidance Maneuvers C, D, and E can be calculated from the following equation: design speed (mph) total pre-maneuver and maneuver time (seconds) Copyright 2013 Gregory J. Taylor, P.E. Page 8 of 32

9 Passing Sight Distance Passing sight distance is the length of roadway needed for drivers on two-lane two-way highways to complete a normal passing maneuver. Minimum values for passing sight distances are based on warrants for no-passing zones on two-lane highways as presented in the Manual on Uniform Traffic Control Devices (MUTCD). Potential conflicts in passing operations are ultimately determined by driver response to: o View of roadway ahead o Passing and no-passing markings Horizontal alignment is also considered when deriving the location, extent, and percentage of these passing distances. Research has shown that more sight distance is needed for passing maneuvers than for stopping sight distance which is continuously provided for along roadways. The following table shows design values for passing sight distance on two-lane highways. Passing Sight Distance - Two-Lane Roadways Design Assumed Speed (mph) Passing Sight Speed Passed Passing Distance (mph) Vehicle Vehicle (ft) Source: AASHTO Green Book Table Copyright 2013 Gregory J. Taylor, P.E. Page 9 of 32

10 Assumptions Regarding Driver Behavior Passing and opposing vehicle speeds are equal to the roadway design speed Speed differential between passing and passed vehicle is 12 mph Passing vehicle has adequate acceleration capability to reach speed differential (40% of way through passing maneuver) Vehicle lengths are 19 feet Passing driver s perception-reaction time to abort passing maneuver is 1 second Deceleration rate of 11.2 ft/s² for passing vehicle when passing maneuver is aborted Space headway between passing and passed vehicles is 1 second Minimum clearance between passing and opposed vehicles upon return to normal lane is 1 second Passing sight distances should be equal or greater than the minimum values. Also, they should be as long and frequent as possible, depending on: topography design speed cost intersection spacing Intersection Sight Distance Intersection sight distance is the length of roadway along the intersecting road for the driver on the approach to perceive and react to the presence of potentially conflicting vehicles. Drivers approaching intersections should have a clear view of the intersection with adequate roadway to perceive and avoid potential hazards. Sight distance should also be provided to allow stopped vehicles a sufficient view of the intersecting roadway in order to enter or cross it. Intersection sight distances that exceed stopping sight distances are preferable along major roads to enhance traffic operations. Methods for determining intersection sight distances are based on many of the same principles as stopping sight distance. Copyright 2013 Gregory J. Taylor, P.E. Page 10 of 32

11 Source: CTRE Iowa State University Sight triangles are areas along intersection approach legs that should be clear of obstructions that could block a driver s view. The dimensions are based on driver behavior, roadway design speeds, and type of traffic control. Object height is based on vehicle height of 4.35 feet. Copyright 2013 Gregory J. Taylor, P.E. Page 11 of 32

12 Source: CTRE Iowa State University Copyright 2013 Gregory J. Taylor, P.E. Page 12 of 32

13 Recommended sight triangle dimensions vary for the following different types of traffic control: Case A: Intersections with no control Case B: Intersections with stop control on the minor road o Case B1: Left turn from the minor road o Case B2: Right turn from the minor road o Case B3: Crossing maneuver from the minor road Case C: Intersections with yield control on the minor road o Case C1: Crossing maneuver from the minor road o Case C2: Left or right turn from the minor road Case D: Intersections with traffic signal control Case E: Intersections with all-way stop control Case F: Left turns from the major road Copyright 2013 Gregory J. Taylor, P.E. Page 13 of 32

14 VERTICAL ALIGNMENT Roadway Vertical Alignments A proposed roadway must consider the existing constraints while balancing safety and economic factors. This usually produces roads that are not flat or straight with vertical and horizontal curves to accommodate local conditions (topography, property owners, land usage, natural resources, cost, environment, etc.). A roadway s vertical alignment is comprised of crest and sag curves, and the straight grades connecting them. This alignment profile is typically shown as a graph with elevation on the vertical axis with horizontal centerline distance as the horizontal axis. Geometric design of this proposed roadway profile is related to safety, vehicle operations, drainage, and construction issues. The type of terrain to be traversed plays a major role in the alignment of roadways particularly the vertical profile. The variations in topography can be classified by its general character into the following categories: Level: Both horizontal and vertical sight distances are generally lengthy Rolling: Natural slopes rise or fall below the roadway with occasional steep slopes Mountainous: Abrupt changes in ground elevation with respect to the roadway Rolling terrain typically generates steeper grades than level terrain which causes significant reduction in truck speeds. For mountainous terrain, this reduction will be more drastic. For level terrain, design controls can be satisfied with little or no construction difficulty or expense. As the terrain becomes more challenging, more complex construction methods must be used to integrate the proposed alignment with the existing ground. The vertical alignment should be coordinated with the existing topography, available right-of-way, utilities, developments, and drainage patterns. Tangent Grades Roadway design should encourage uniform operation throughout the proposed transportation facility. Most passenger cars can readily negotiate grades as steep as 4 Copyright 2013 Gregory J. Taylor, P.E. Page 14 of 32

15 to 5 percent without a significant loss in speed. However, vehicle speeds decrease progressively with grade increases. Vertical grades have a much greater effect on truck speeds. The speed of traffic is often controlled by heavy vehicles on uphill grades due to restrictive sight distances. Lengthy grades greater than 3% start to influence passenger car speeds while shorter, steeper grades affect truck speeds. Although the average speeds of trucks and passenger cars are similar for level roadway sections, trucks usually increase downgrade speeds up to 5% and decrease upgrade speeds by 7%. Their maximum speed is dependent on the length/steepness of the grade and the truck s weight/power ratio (gross vehicle weight divided by the net engine power). Travel times and speeds for trucks are byproducts of their weight/power ratio with present acceptable values for highway users of approximately 200 lb/hp. These values have steadily decreased over the years resulting in greater power and better climbing ability on upgrades. Normally, vertical grades should be less than the maximum design grade. Maximum values should be used rarely, if possible. Design guidelines for maximum grades have been established from grade controls presently in use. Design Speed Maximum Grade 70 mph 5% 30 mph 7 to 12% (depending on terrain) For major routes, maximum grade values of 7 to 8 percent are typical for 30 mph design speed. However, for one-way downgrades less than 500 ft long, the maximum grade should be approximately 1% steeper than other locations. The maximum may be 2% steeper for low-volume rural highways. On the hand, maintaining adequate minimum grades can be a primary concern for many roadway locations. It is provided for drainage purposes and depends on rainfall, soil type, and site conditions. For roadways with adequate cross slopes for surface drainage, a typical value for minimum grade is 0.5 percent. Stormwater drainage systems (pipes, inlets, swales, channels, etc.) should be considered when using flat grades to keep the spread of water within acceptable limits. The vertical profile may Copyright 2013 Gregory J. Taylor, P.E. Page 15 of 32

16 affect road drainage by creating very flat roads/sag curves that may have poor drainage, or steep roads with high velocity flows. 0.3 percent Minimum control in some states 0.15 percent Practical minimum for flat terrain 0 percent (flat) Should be avoided relies totally on roadway crossslope for drainage Critical Length of Grade: maximum length of upgrade on which a loaded truck may operate without an unacceptable speed reduction The following data may be used to determine values for critical lengths of grade: Size, power, and gradeability data for design vehicle A typical loaded truck used as a design control has a weight/power ratio of 200 lb/hp Entrance speeds to critical length The average running speed can be used for vehicle speeds at the beginning of an uphill approach Minimum tolerable speeds of trucks on upgrades Roadways designs should strive to prevent intolerable truck speed reductions for following drivers Research has shown that vehicles deviating from the average roadway speed stand a greater chance of crashing. Climbing Lane: exclusive additional lane for slow-moving vehicles (uphill) which permits other vehicles to use the normal roadway lanes for passing. These lanes may be used for locations with a level of service or truck speed is much less on an upgrade versus the approach. Copyright 2013 Gregory J. Taylor, P.E. Page 16 of 32

17 Justification Criteria for Climbing Lanes o Upgrade traffic flow rate exceeds 200 vehicles per hour o Upgrade truck flow rate exceeds 20 vehicles per hour o One of the following exists: 10 mph or greater speed reduction expected by heavy trucks Level of service of E or F Reduction of 2 or more levels of service from approach segment to grade Climbing lanes are an inexpensive way to overcome capacity reductions, improve operation in truck congestion areas, and reduce crashes. For existing roads, climbing lanes may defer reconstruction. On new projects, they can increase operational efficiency without creating a multilane highway. Successful methods for increasing passing opportunities on 2-lane roads include: passing lanes; turnouts; shoulder driving; and shoulder use sections. A passing lane is an added lane for improving vehicle traffic in low capacity sections. The optimal lane length is typically 0.5 to 2 miles (longer lane lengths for higher traffic volumes). Transition tapers at the ends of added-lane sections can be designed from the following equations: 45 mph or greater Less than 45 mph width (feet) Copyright 2013 Gregory J. Taylor, P.E. Page 17 of 32

18 A minimum sight distance of 1000 feet is recommended for taper approaches. Optimal Passing Lane Lengths One-Way Passing Lane Flow Rate Length (veh/h) (mile) 100 to to to to to to to 2.00 Source: AASHTO Green Book Table 3-31 Passing Lane Design Procedure 1. Both horizontal and vertical alignments should provide as much passing sight distance as possible 2. The level of service may be reduced as design volume approaches capacity 3. Additional climbing lanes may be needed where the critical length of grade is less than the length of an upgrade 4. If passing opportunities are still restricted after applying the previous measures, passing lanes may still be considered The 2+1 roadway concept consists of a continuous 3-lane roadway striped for passing lanes in alternating directions. This type of road is typically used at locations where a four-lane roadway is considered impractical (environmental/economic constraints, traffic volume requirements, etc.). It generally produces a level of service at a minimum of 2 levels higher than typical two-lane highways. A 2+1 road should be avoided where flow rates exceed 1200 vehicles/hr per one direction of travel (4-lanes are better for high flow rates). It should also be limited to level and rolling terrain. Copyright 2013 Gregory J. Taylor, P.E. Page 18 of 32

19 High volume intersections and driveways are crucial factors for determining passing lane locations (proper location will minimize the number of turning movements). Sight Distance Stopping Decision 2+1 Roadway Location Provided continuously Intersections & lane drops A turnout is a widened, unobstructed shoulder area for slow-moving vehicles to pull out of traffic. A slow vehicle is expected to pull over in the turnout long enough for passing vehicles to pass before returning to the through lane. Turnouts are typically used on lower volume roads or steep grades (mountains, beaches, scenic areas with a minimum of 10% heavy vehicle traffic). Typical entry & exit taper lengths: 50 to 100 feet Minimum turnout width: 12 feet (16 feet desirable) Minimum sight distance: 1000 feet on approach Shoulder driving is the practice of slow-moving vehicles driving on the shoulder when approached from the rear by another vehicle, and returns to the roadway after being passed. This custom occurs at locations with adequate paved shoulders that function as continuous turnouts. Shoulder widths should be a minimum of 10 feet (12 feet desirable). Shoulder use sections are road segments where slow vehicles are permitted to use paved shoulders at specific signed sites (similar to shoulder driving). This application is a more limited use of paved shoulders with lengths ranging from 0.2 to 3 miles with minimum widths of 10 feet (preferably 12 feet). Copyright 2013 Gregory J. Taylor, P.E. Page 19 of 32

20 Emergency Escape Ramps Emergency escape ramps at appropriate locations (long descending grades, topographic needs, etc.) can provide an area for out-of-control vehicles to decelerate and stop away from roadway traffic. Effective escape ramps help save lives and reduce property damage by providing acceptable deceleration rates and good driver control. These ramps should be designed for the worst case scenario, where the speeding vehicle is out of gear with brake system failure. The minimum entering speed for emergency escape ramps is 80 mph (90 mph preferred). Typical Causes of Out-of-Control Vehicles Brakes: overheating or mechanical failure Transmission: failure to downshift properly, etc. Emergency escape ramps can be classified into the following major categories: Gravity (least desirable) stops forward motion but does not prevent rollback Sandpile (less desirable) severe deceleration characteristics and weather affects Arrester bed Within these categories, there are four basic ramp designs - sandpile, descending grade, horizontal grade, and ascending grade (most common). Copyright 2013 Gregory J. Taylor, P.E. Page 20 of 32

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22 Guidelines for Effective Escape Ramps Ramp length should be adequate for dissipating the vehicle s kinetic energy Ramp alignment should be tangent or flat curvature Ramp width should be sufficient to hold more than one vehicle (minimum width of 26 feet, 30 to 40 feet desirable) Arrester bed aggregate should be clean, rounded, uncrushed, similar size, and free of fine-sized material (AASHTO No. 57 is effective) Arrester beds should have a minimum aggregate depth of 3 feet with a recommended depth of 42 inches (3.5 feet) Arrester beds should be graded for sufficient drainage to prevent freezing or contamination Ramp entrances should be designed for vehicles operating at high speeds (provide as much sight distance as possible) Exit signage should clearly indicate the ramp s access and allow the driver adequate time to react A service road (10 ft minimum width) should be located adjacent to the arrester bed for tow truck/maintenance vehicle access Anchors should be located at 150 to 300 foot interval adjacent to the arrester bed for tow truck operations one anchor installed 100 feet in advance of the bed VERTICAL CURVES A road s vertical alignment consists of road slopes (grades) connected by vertical curves. Parabolic vertical curves provide a gradual change from one grade to another for vehicles to smoothly navigate any grade changes. Normally, a vertical curve with an equivalent vertical axis is centered on the Vertical Point of Intersection (VPI). These curves are either classified as sag (concave) or crest (convex). Parabolic Curve Advantages Straightforward computations Good riding comfort Easy field implementation Design guidelines for vertical curve lengths are generally based on providing for sufficient sight distance and driver comfort. Longer stopping sight distances should be used where possible to ease any shock from grade changes. Copyright 2013 Gregory J. Taylor, P.E. Page 22 of 32

23 Terms A = algebraic difference in grades (percent) VPC = begin of vertical curve VPT = end of vertical curve G1 = initial roadway (tangent) grade G2 = final roadway (tangent) grade h 1 = Height of eye above roadway surface h 2 = Height of object above roadway surface L = vertical curve length VPI = point of vertical interception (intersection of initial and final grades) The K value is the horizontal distance needed to create a one-percent change in grade. It is a measure of curvature that is expressed as the ratio of the vertical curve length to the algebraic difference in the grades (L/A). This is useful in determining the horizontal distance from the Vertical Point of Curvature (VPC) to the sag or crest points. The K value can also be useful for determining minimum vertical curve lengths. SAG VERTICAL CURVES Sag vertical curves have a tangent slope at the curve end which is higher than at the beginning. Sag curves appear as valleys by first going downhill, reaching the bottom of the curve, and continuing uphill (resembling a concave curve). Copyright 2013 Gregory J. Taylor, P.E. Page 23 of 32

24 Sag Vertical Curve Criteria Headlight sight distance Passenger comfort Drainage control General appearance The most important determinant of sag curve length is headlight sight distance which is limited by the headlight position and direction when traveling at night. This distance must be adequate for a driver to see an obstruction and stop within the headlight sight distance. The sag curve length should be of sufficient for the light beam to illuminate the roadway sufficiently to provide stopping sight distance. This permits drivers to see the roadway ahead. Headlight Sight Distance Assumptions Headlight height: 2 feet Upward divergence of light beam from longitudinal axis of vehicle: 1 degree algebraic difference in grades (percent) The vertical curve length needed to satisfy passenger comfort is typically 50% of the headlight sight distance for normal conditions. A good rule-of-thumb approximation for minimum sag vertical curve length is 100A or K = 100 feet per percent change in grade. Copyright 2013 Gregory J. Taylor, P.E. Page 24 of 32

25 CREST VERTICAL CURVES Crest vertical curves have a lower tangent slope at the end of the curve than at its beginning. Crest vertical curves are convex upwards and typically appears as a hill, with vehicles first going uphill, reaching the top of the curve and then continuing downhill. Crest curves are formed in locations where two grades meet in any of the following conditions: 1. one positive grade meets another positive grade 2. a positive grade meets a flat grade 3. an ascending grade meets a descending grade 4. a descending grade meets another descending grade A crucial design criterion for these curves is stopping sight distance (the distance visible over the curve crest). This distance is determined by the speed of roadway traffic. Crest curves need to be coordinated with the horizontal alignment. Horizontal curves should not be located beyond the vertical crest in a way that prevents the driver from seeing upcoming changes in the horizontal alignment. The appropriate equation depends on the vertical curve length versus available sight distance. Copyright 2013 Gregory J. Taylor, P.E. Page 25 of 32

26 Source: CTRE Iowa State University The following AASHTO equations are the basic formulas for determining crest vertical curve lengths in terms of algebraic grade difference (A) and sight distance (S). When When algebraic difference in grades (percent) Copyright 2013 Gregory J. Taylor, P.E. Page 26 of 32

27 Stopping Sight Distance Criteria Height of Eye: 3.50 feet Height of Object: 2.00 feet Using the typical values for eye height and object height, the calculations for crest vertical curve lengths are simplified into the following equations. When When algebraic difference in grades (percent) Copyright 2013 Gregory J. Taylor, P.E. Page 27 of 32

28 Crest vertical curve design values for passing sight distance differ from stopping sight distance due to sight distance and object height differences and may be determined from the following AASHTO formulas. When When algebraic difference in grades (percent) Using passing sight distance criteria, minimum crest curve lengths are much longer than those for stopping sight distance. Typically, it is not realistic to use passing sight distance controls due to high costs of crest cuts and difficulty of integrating long vertical curves to the topography. Passing sight criteria for crest curves may be appropriate for: Low speed roadways with gentle grades High speeds with small grade differences Locations not needing significant grading Copyright 2013 Gregory J. Taylor, P.E. Page 28 of 32

29 Crest Vertical Curve Design Controls Design Passing Sight Rate of Speed Distance Vertical Curvature (mph) (ft) K Design Source: AASHTO Green Book Table 3-35 General Controls for Vertical Alignments Smooth, gradual gradeline consistent with roadway type and terrain desired Avoid hidden dips/changes in the roadway profile Evaluate any proposed profile containing substantial momentum grades with traffic operations Avoid broken back (consecutive vertical curves in the same direction) gradelines Place steep grades at the bottom and flatter grades near the top of ascents Reduce grades through at-grade intersections with moderate to steep grades Avoid sag curves in cut sections, where possible Copyright 2013 Gregory J. Taylor, P.E. Page 29 of 32

30 COORDINATION OF HORIZONTAL AND VERTICAL ALIGNMENTS Roadway geometry influences its safety performance. Crash research has shown that roadway factors are the second most contributing factor to road accidents. Crashes tend to occur more frequently at locations with sudden changes in road character (example: sharp curves at the end of long tangent roadway sections). The concept of design consistency compares adjacent road segments and identifies sites with changes that might violate driver expectations. Design consistency analysis can be used to show decreases in operating speed at curves. The horizontal and vertical geometrics are the most critical design elements of any roadway. These alignments should be designed concurrently to enhance vehicle operation, uniform speed, and aesthetics without additional costs (examples: checking for additional sight distance prior to major changes in the vertical alignment; or revising design elements to eliminate potential drainage problems). The horizontal and vertical alignment designs complement each other and poor designs can reduce the quality of both. Computer-aided design (CAD) is a common method used to produce an optimal three-dimensional design. Design speed helps to determine roadway location and keeps all design elements (traffic, topography, geotechnical concerns, culture, future development, project limits, etc.) in balance. It limits many design values (curves, sight distance) and influences others (width, clearance, maximum gradient). AASHTO Design Guidelines for Horizontal and Vertical Alignments Vertical and horizontal elements should be balanced. A desirable design which optimizes safety, capacity, operation, and aesthetics within the location s topography. Horizontal and vertical alignment elements should be integrated to provide a pleasing facility for roadway traffic. Avoid sharp horizontal curves near the top of a crest vertical curve or near the low point of a sag vertical curve. Using higher design values (well above the Copyright 2013 Gregory J. Taylor, P.E. Page 30 of 32

31 minimum) for design speed can produce suitable designs and meet driver s expectations. Horizontal and vertical curves should be flat as possible for intersections with sight distance concerns. For divided roadways, it may be suitable to vary the median width or use independent horizontal/vertical alignments for individual one-way roads. Roadway alignments should be designed to minimize nuisance in residential areas. Typical measures may include: depressed facilities (decreases facility visibility and noise); or horizontal adjustments (increases buffer zones between traffic and neighborhoods). Horizontal and vertical elements should be used to enhance environmental features (parks, rivers, terrain, etc.). Roadways should lead into outstanding views or features instead of avoiding them where possible. Exception Long tangent sections for sufficient passing sight distance may be appropriate for twolane roads needing passing sections at frequent intervals. Copyright 2013 Gregory J. Taylor, P.E. Page 31 of 32

32 REFERENCES A Policy on Geometric Design of Highways and Streets, 6 th Edition AASHTO. Washington, D.C Note: This text is the source for all equations and tables contained within this course, unless noted otherwise. Handbook of Simplified Practice for Traffic Studies Center for Transportation Research & Education Iowa State University. Ames, Iowa Manual on Uniform Traffic Control Devices (MUTCD) Federal Highway Administration. Washington, D.C Traffic Engineering Handbook, 5 th Edition Institute of Transportation Engineers. Washington, D.C Copyright 2013 Gregory J. Taylor, P.E. Page 32 of 32