This Technical Memorandum extends the expiration date of Technical Memorandum TS-07 to August 1, 2021 unless superseded prior to that date.

Transcription

1 Minnesota Department of Transportation Engineering Services Division Technical Memorandum No TS-06 August 1, 2018 Technical Memorandum To: Electronic Distribution Recipients From: Nancy T. Daubenberger, P.E. Division Director, Engineering Services Subject: Expiration and Purpose This Technical Memorandum extends the expiration date of Technical Memorandum TS-07 to August 1, 2021 unless superseded prior to that date. The purpose of this Technical Memorandum is to update the MnDOT traveled lane width design criteria and will provide for more design flexibility when selecting a traveled lane width. Guidelines Questions Refer to Attachment TM TS-07 Any questions regarding the technical provisions of this Technical Memorandum can be addressed to the following: Douglas Carter, P.E., State Geometrics Engineer, MnDOT, at (651) Jim Rosenow, P.E., Design Flexibility Engineer, MnDOT, at (651) Any questions regarding publication of this Technical Memorandum should be referred to the Design Standards Unit, DesignStandards.DOT@state.mn.us. A link to all active and historical Technical Memoranda can be found at To add, remove or change your name on the Technical Memoranda mailing list, please visit the web page Attachments: TM TS-07 -END-

2 MINNESOTA DEPARTMENT OF TRANSPORTATION Engineering Services Division Technical Memorandum No TS-07 To: From: Subject: Electronic Distribution Recipients Jon M. Chiglo, P.E. Division Director, Engineering Services Expiration This Technical Memorandum supersedes TM TS-02 and will remain in effect until September 12, 2018 unless superseded or published in the MnDOT Road Design Manual prior to that date. Implementation The design guidance contained in this Technical Memorandum is effective immediately for projects in the early stages of the preliminary design phase, and may be incorporated into projects in a more advanced design phase. It is required that the final selection of the traveled lane width be thoroughly documented in the district project design memo. Introduction Many state transportation departments have been turning to flexible design as a solution to resolving various transportation challenges. The benefits of flexible design, allow for a greater sensitivity to the design needs of multiple travel modes, the local community, and the surrounding environment. This design approach also provides an opportunity to increase safety on a system-wide basis by stretching available funding to improve safety over a larger exposure area. MnDOT has been moving forward with its own flexible design initiative and this Technical Memorandum is one in a series that are being published to help support the statewide effort. MnDOT traveled lane width standards were reviewed with respect to current AASHTO standards and guidance, as well as the Department s experience related to successful projects where the principles of context-sensitive solutions are practiced. The new design criteria presented within this Technical Memoranda represents a conceptual change in how lane width design choices are made. The designer is given more leeway in selecting the most appropriate standard by incorporating a multitude of design considerations. Because of this flexibility, thorough decision documentation will be required. Purpose The purpose of this Technical Memorandum is to update the MnDOT traveled lane width design criteria. This update will provide for more design flexibility when selecting a traveled lane width. Guidelines Lane width influences the comfort, operational characteristics and, in some cases, likelihood of crashes for all users of the public right of way. Lane width selection is a key decision on a project or particular roadway, and due consideration is necessary. Tables 4-3.XX and 4-3.YY and the subsequent guidance provide standard dimensions and considerations, respectively, for selecting design values. The widths given do not include shoulders or curb reaction dimensions. Urban, Suburban and Rural characterize the facility s setting, not necessarily the presence or absence of curbing.

3 Page 2 FUNCTIONAL CLASSIFICATION NOTE: The tabular information must not be applied without thorough analysis using the guidance that follows. This is true even if only a singular value is presented, as a non-standard solution may be more appropriate. Table 4-3.XX TRAVEL LANE WIDTHS RURAL HIGHWAY SETTINGS DESIGN LANE WIDTHS FOR SPECIFIED DESIGN ADT, feet SPEEDS (mph) under to to 2000 over (1) (2) COLLECTOR (1) (2) (2) (2) ARTERIAL (2) (2) FREEWAY (1) (2) 9 feet minimum for roads with a design speed of 40 mph or lower and with a design ADT less than 250 On reconstruction projects, existing 11-foot lanes may be retained where the horizontal alignment is satisfactory and there is no crash pattern suggesting the need for widening Table 4-3.YY TRAVEL LANE WIDTHS URBAN AND SUBURBAN HIGHWAY AND STREET SETTINGS LANE WIDTHS FOR SPECIFIED DESIGN SPEED RANGES, feet FUNCTIONAL CLASSIFICATION LOW SPEED (< 50 mph) HIGH SPEED ( 50 mph) COLLECTOR (1) (2) MINOR ARTERIAL (2) PRINCIPAL ARTERIAL FREEWAY N/A 12 (1) (2) 12 feet may be considered in industrial areas 11 feet minimum on four-lane undivided facilities General Design Considerations The criteria provided are based on the general premise of providing a lane width suitable for the traffic demand including intended operating speeds and the volume of large vehicles. In establishing policy, functional classification and design speed are useful surrogates for demand, but those associations are inherently general and cannot reflect every performance factor for every mode. For that reason, flexibility is provided to allow design values that address actual demands as well as balance potentially competing considerations.

4 Page 3 Care must be exercised in selecting travel lane widths. As with other engineering judgments, an appropriate design balances operational performance with physical, contextual, environmental and economic considerations. Additionally, the safety and operations of the various travel modes along and across the right of way must be equitably balanced amongst themselves. Non-standard design values outside the recommended ranges may occasionally be necessary to realize balance and contextual fit. Rural Highways Lane widths toward the higher end of standard practice are typically desirable, as they provide more generous clearance between opposing vehicles as well as to the shoulder and roadway edge. Narrower standard values are available for lower-speed and lower demand conditions, where the incremental cost of wide lanes can be difficult to justify, or on corridors where site conditions or constraints create particularly high construction costs or environmental impacts. The crash effects of lane width provided in Section of the AASHTO Highway Safety Manual (HSM) may be useful in assessing situational benefits or comparing design alternatives. In areas where bicycles and pedestrians are common and speed reduction is desired, the benefits of narrower lanes stated in the urban and suburban discussion and elsewhere in this document should be taken into account. Urban and Suburban Highways and Streets Special care is demanded for design in urban and suburban environments, where often limited space must be balanced between the various transportation modes and among geometric design elements. Lane width is particularly important on multi-lane streets, where even small variations in design values are multiplied across the cross section. One research study has found that narrower lanes on urban and suburban streets were generally associated with lower crash frequencies compared to wider lanes. (An exception to this is four-lane undivided streets, where 9- and 10-foot lanes have been associated with higher crash frequencies than wider lanes respectively.) Furthermore, narrower cross sections reduce crossing distances and have been associated with reduced travel speeds, both direct factors in the safety of pedestrians. For these reasons, lane widths on urban and suburban streets should be designed no wider than to adequately accommodate the vehicular traffic volume and composition. General design guidance is as follows: Lane widths of 11 feet are a good fit for a wide variety of urban arterials and collectors. 11-foot lanes are fully adequate for vehicular operation on low-speed facilities and can be thought of as roughly equivalent in terms of comfort and usability to 12-foot lanes on high-speed roads and streets. 11-foot lanes are also appropriate on high-speed facilities under favorable geometric conditions. Lane widths of 10 feet are typically most suitable where truck and bus volumes are relatively low and design speeds are 35 mph or less, as well as in more constrained circumstances. Lane widths of 12 feet tend to be most applicable in high-speed or high-demand circumstances. On low-speed facilities, the use of 12-foot lanes should be limited to very high demand conditions or to limited-access roadways, so as to avoid wherever practical the drawbacks often associated with overly wide cross sections, such as excessive speed and longer pedestrian exposure. Unequal Lane Widths On multi-lane streets, unequal lane widths may be utilized; this design approach can be especially useful to tailor widths to specific needs in constrained cross sections. For example, locating the wider lane to the outside provides more space for large vehicles and bicycles, both of which are more likely to occupy that lane.

5 Page 4 Shared or Wide Outside Lane Where insufficient space exists for a designated bike lane or paved shoulder on an urban or suburban facility, lane widths may be increased from the values provided to allow motor vehicles to share outside lanes with bicycle traffic. Refer to the MnDOT Bikeway Facility Design Manual for guidance and design values. As noted therein, lane widths associated with the "wide outside lane treatment can be associated with undesirable increases in travel speeds. For that reason, it should only be considered where judged beneficial to non-motorized users. Additionally, pavement marking and/or signing treatments highlighting the shared-lane condition should be provided. Specific Design Considerations The following factors should be taken into consideration when selecting lane width values within the ranges in Tables 4-3.XX and 4-3.YY or in determining that a non-standard value may be in order. The design judgment must take into account all applicable factors rather than only one or two factors weighted inequitably. 1. Average daily traffic (ADT) Rural For rural highways, the criteria in Table 4-3.XX reflect the general concept of providing a width related to the traffic demand. Urban and Suburban The Highway Capacity Manual (HCM) states that the saturated flow capacity of a lane through a signalized intersection is uniform for lane widths of 10 feet through 12.9 feet. For lane widths of less than 10 feet, an expected 4 percent reduction in capacity should be taken into account in the design judgment. 2. Trucks and large vehicles For design purposes, a heavy commercial percentage of 10% is considered relatively high, while a rule-of-thumb threshold for moderate truck volume is 5%. Trucks should generally be weighted as a design consideration based on where their volume lies with respect to these percentages. Designated Truck Routes Federal and State Designated Truck Routes (e.g., National Truck Network, Minnesota Twin Trailer Network, Oversize/Overweight (OSOW) Super Load Corridor Network, house moving corridors)are intended to accommodate large truck travel. This designation should not affect lane width selection, as such vehicles represent a very small percentage of overall traffic. Instead, an overall roadway width sufficient to accommodate large vehicles effectively on these routes should be provided. Urban and Suburban As stated above, lane widths of 11 feet should be considered fully sufficient for truck traffic at low speeds, but where high truck and bus demand is combined with near-capacity hourly volumes, wider lanes may be advisable to optimize traffic flow. Studies have found little operational difficulty for buses in 10-foot lanes. It is notable, however, that the side mirror-to-mirror width of the intra-city bus vehicle is approximately 10.5 feet. For that reason, where the bus and truck percentage is high, maneuvering space may be occasionally limited, and this factor should be considered in the overall roadway width determination. 3. Design speed The design speed, perhaps more so than any other design control on a highway, will have a major impact on all facets of geometric design, and other design elements. The project segment s appropriate design speed depends upon the functional classification and use, average daily traffic ADT, anticipated and desirable operating speed, terrain, and adjacent land use of the highway. 4. Horizontal alignment elements Horizontal alignment is a complex factor to take account of, as it interrelates with the overall cross section design as well as corridor consistency (see below). The frequency and severity

6 Page 5 of horizontal curves are particularly pertinent on high-speed and rural facilities, largely because of the lateral space demands of large trucks but also due to the tracking behavior of passenger car drivers. On lower-speed or lower-demand roads, some off-tracking by large vehicles that encroach marginally into opposing traveled way may be judged a practical alternative to lane widening. The availability of a surfaced shoulder to accommodate off-tracking should be considered in selecting a lane width; conversely, providing a narrow paved shoulder, intermittently or continuously, can supplement a lane width that may be insufficient for off-tracking through occasional curves. Alternatively, if sharper curves are infrequent, traveled-way widening could be provided only where needed, allowing a narrower typical lane width. Expected bicycle use needs to be taken into account as well, especially where shoulders are assumed to accommodate off-tracking. In any case, vehicle swept path analyses should be performed using the design vehicle(s), and the frequency of usage by larger vehicles must be taken into account in deciding to what extent to accommodate them. 5. Corridor consistency The goal of providing a consistent operating environment for the user must be balanced against the objective of fitting lane width to need and context. For any project, an analysis should be made of the broader corridor using logical segment limits so that present and long term improvements are well coordinated to the extent foreseeable. The goal should be a user experience, in both interim and ultimate conditions, that makes sense from each travel mode s perspective and reasonably conforms to expectation. To attain this goal, it may be desirable, for example, to vary lane width over time through planned restriping as projects that alter lane width are constructed as part of a corridor improvement. 6. Interrelationships with other cross sectional elements Although lane width selection is largely related to traveled-way demand, the judgment is incomplete without consideration of the overall cross section and adjacent design elements in light of total function and safety. Rural In some cases, a narrower lane width may enable a wider shoulder width, offering a lower likelihood of run-off-the-road crashes and more space for non-motorized traffic. Tradeoffs of this sort are particularly pertinent on preservation projects or when roads must be reconstructed within an existing road bed. Urban and Suburban Adjacent elements should be assessed based on their respective functions and needs. For example, while travel lane width may be based in large part on traffic volume and composition, the adjacent bicycle or parking lane width is typically based on necessary operational space for an individual user. A holistic approach to the overall cross section is vital to designing roadways for performance. 7. Space constraints Rural, suburban and urban settings can present constrained environments. Space limitation and the need to apportion it among competing functions can sometimes be an overriding factor in lane width selection. The operational and safety consequences associated with various values of each cross sectional element must be evaluated for each function/user to assess tradeoffs and arrive at an equitable balance.

7 Page 6 8. Multimodal considerations Mobility and safety apply to all modes of travel in the right of way. The ability of non-motorized users to travel along and across streets and highways safely and comfortably is a principal measure of livability. As discussed earlier in this section, the lane width selection for urban and suburban streets will often significantly affect vehicular operating speeds and the distances pedestrians face in crossing roadways. Bicycles and pedestrians are among the most vulnerable roadway users, and vehicular speed and exposure are the two primary factors in non-motorized users crash frequency and severity. 9. Context MnDOT s overarching design policy is to balance the objectives of mobility and safety for all travel modes with preservation and enhancement of aesthetic, scenic, historic, cultural, environmental and community values. When designing a road or street, understanding these and other aspects of a facility s context is necessary to make design judgments that are sensitive to its built and natural environments. 10. Maintenance Geometric design choices can have a great impact on the ease of maintainability for any facility. The responsible operations and maintenance organization must be consulted with and have the ability to provide input. This must occur prior to the finalization of any cross-sectional elements (e.g. lane width, shoulder width, bike lanes, bus shoulders, etc.). Questions Any questions regarding the technical provisions of this Technical Memorandum can be addressed to any one of the following: Michael Elle at (651) , Design Standards Engineer Jim Rosenow at (651) , Design Flexibility Engineer Darwin Yasis at (651) , State Geometrics Engineer Any questions regarding publication of this Technical Memorandum should be referred to the Design Standards Unit, DesignStandards.DOT@state.mn.us. A link to all active and historical Technical Memoranda can be found at To add, remove, or change your name on the Technical Memoranda mailing list, please visit the web page -END-