Minnesota Department of Transportation Engineering Services Division Technical Memorandum No. 17-12-TS-05 Technical Memorandum To: Electronic Distribution Recipients From: Nancy T. Daubenberger, P.E. Divison Director, Engineering Services Subject: Expiration This Technical Memorandum supersedes TM 12-12-TS-06 and will remain in effect until December 5, 2022 unless superseded or published in the MnDOT Road Design Manual prior to that date.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 and shoulder widths 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 shoulder 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 Memorandum represents a design process change in the determination of shoulder width. Designers are given more leeway in selecting the most appropriate standard by incorporating a multitude of design considerations. Because of this flexibility, detailed decision documentation is required. Purpose The purpose of this Technical Memorandum is to update the MnDOT shoulder width design criteria. This update will provide for more design flexibility in roadway shoulder design.
Guidelines Technical Memorandum No. 17-12-TS-05 Page 2 Roadway shoulders have a profound impact on overall roadway operations, safety, maintenance, and multimodal users. Shoulder width selections are a key decision on any roadway, and due consideration is necessary. Tables 1 through 3, along with the subsequent guidance, provide standard dimensions and considerations, respectively, for determining appropriate design values. 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 1 MINIMUM SHOULDER WIDTHS COLLECTORS MINIMUM WIDTH (FEET) (1,6) HIGHWAY TYPE Outside (or Right) Usable (2) Paved (3,4,5) ADT < 400 2 0 Collector (Rural) ADT 400-1500 5 (7) 1 ADT 1500-2000 6 1 ADT > 2000 8 2 ADT < 400 Curb Reaction (11) > 45 mph 2 (8) ADT 400-1500 Curb Reaction (11) Collector (Urban / Suburban) ADT 1500-2000 > 45 mph 5 (7,8) > 45 mph 6 (8) Curb Reaction (11) ADT >2000 Curb Reaction (11) > 45 mph 8 (8,9) Parking - Residential 7-8 (8,9,10) Parking - Commercial 8-10 (8,9,10) (1) For the purposes of design documentation and design exceptions, the usable width is considered the controlling criterion. (2) Usable width is measured to the shoulder P.I. (3) Maximum width is 1.5 feet less than the usable width.
Page 3 (4) To adequately accommodate bicycles and pedestrians outside the travel lane, a minimum paved width of 4-feet exclusive of rumble strips is necessary. Refer to the AASHTO Guide for the Development of Bicycle Facilities, The MnDOT Bikeway Facility Design Manual, and the MnDOT Road Design Manual. (5) Where pedestrian facilities are to be accommodated on the shoulder, refer to the AASHTO Guide for the Planning, Design, and Operation of Pedestrian Facilities, and of the MnDOT Road Design Manual. (6) Where roadside barriers are included on rural roads, a minimum offset of 4 feet from the traveled way to the barrier should be provided, wherever practical. Where bicycles and pedestrians are to be accommodated, a 5 foot minimum offset is recommended. (7) Shoulder width may be reduced provided that a minimum roadway width of 30 feet is maintained. (8) Where curb and gutter sections are used, the gutter pan width may be considered as part of the shoulder or parking lane width, but not part of a bicycle and pedestrian access width. (9) ADA guidelines concerning parking must be taken into consideration. Refer to the MnDOT ADA guidelines. (10) Parallel parking may be considered as long as the capacity provided by the through lanes is not unduly restricted. However, parking is highly undesirable on roadways with a design speed greater than 45 mph. (11) Refer to Table 4; Minimum Curb Reaction Distance. Table 2 MINIMUM SHOULDER WIDTHS ARTERIALS MINIMUM WIDTH (FEET) (1) HIGHWAY TYPE Median (or Left) Outside (or Right) (10) Usable (2) Paved Usable (2) Paved (3,4) ADT < 400 4 (5) 2 Lanes ADT 400-2000 6 (5) 2 Arterials (Rural) ADT > 2000 8 (5) Divided 4-lanes 5.5 (6) 4 (5) 9.5 (6) 8 Divided 6-lanes 9.5 (6) 8 (5) 9.5 (6) 8 With Parking (9) 7-10 2 Lanes Without Parking Curb Reaction (7) > 45 mph Without Parking 8 Arterials (Urban / Suburban) (8) With Parking (9) 7-10 4 + Lanes Without Parking Curb Reaction (7) > 45 mph Without Parking 8 Divided (4 or more lanes) Curb Reaction (7) (See Above) (1) In cases where a wall or median barrier is adjacent to the shoulder, the AASHTO Roadside Design Guide should be consulted for guidance in selecting additional lateral clearance from the edge of the traveled way to the
Page 4 base of the wall or barrier. Where bicycles and pedestrians need to be accommodated, a 5 foot minimum paved offset is recommended. (2) Usable width is measured to the shoulder P.I. (3) Maximum width is 1.5 feet less than the usable width. (4) To accommodate bicycles and pedestrians outside the travel lane, a minimum paved width of 4 feet exclusive of rumble strips is necessary. Refer to the AASHTO Guide for the Development of Bicycle Facilities, The MnDOT Bikeway Facility Design Manual, and the MnDOT Road Design Manual. (5) Controlling criterion for the purposes of design documentation and design exceptions. (6) Paved width plus 1.5 feet. (7) Refer to Table 4; Minimum Curb Reaction Distance. (8) Parallel parking is not recommended on roadways with a design speed greater than 45 mph. (9) ADA guidelines concerning parking must be taken into consideration. Refer to the MnDOT ADA guidelines. (10) Where pedestrian facilities are to be accommodated on the shoulder, refer to the AASHTO Guide for the Planning, Design, and Operation of Pedestrian Facilities, and the MnDOT Road Design Manual. Table 3 MINIMUM SHOULDER WIDTHS INTERSTATES AND FREEWAYS HIGHWAY TYPE Median (or Left) WIDTH (FEET) (1) Outside (or Right) Paved Paved (1) Interstates / Freeways 4-Lanes 4 10 (3) 6-Lanes 10 (2)(4) 10 (3) 8-Lanes or More 10 (2)(5) 10 (5) (1) Where truck traffic exceeds 250 DDHV, a paved (right) shoulder width of 12 feet should be considered. (2) Where truck traffic exceeds 250 DDHV (on sections with six or more lanes), a paved (left) shoulder width of 12 feet should be considered. (3) In mountainous type terrain (steep bluff areas of Minnesota), an 8 foot minimum paved right shoulder may be used on a traveled way consisting of four or six lanes. (4) In mountainous type terrain (steep bluff areas of MN), a 4 foot minimum paved left shoulder may be used on a traveled way of four or six lanes. (5) In mountainous type terrain (steep bluff areas of MN), where eight or more lanes are provided, an 8 foot minimum paved shoulder width may be used for both sides. Table 4 Design Speed Standard Curb Reaction Dimensions Curb Reaction Width for Indicated Curb Types (feet) B, V or vertical monolithic D, S or sloped monolithic 1-2 0-2 > 45 mph 2-3 1-3
Page 5 Curb Reaction Design Criteria: 1. Where curb & gutter is constructed, the edge of traveled lane should preferably align with the front lip of concrete gutter to delineate the traveled way and guide pavement marking placement. Additional width is needed when bicycles and pedestrians must travel in the gutter area. 2. Where zero curb reaction is employed, curb should be constructed without gutter or as a monolithic construction. 3. Selection of a design value within the ranges provided should consider all the following factors, as applicable: Where the design speed lies within the respective high- and low-speed ranges. Curb height. Presence of or anticipated future need for traffic barrier (i.e. guardrail) along the curb line. Need for drainage collection structures adjacent to the curb and the dimensions of the grate casting(s). Degree of physical constraint in the cross section. Desire to limit the roadway width in urban environments to reduce vehicular speeds and improve pedestrian safety. 4. Regardless of the design dimensions selected above, the curb reaction values for use in truck turning analyses should always be 1 foot for B and V curbs and 0 feet for D and S curbs. General Design Considerations The criteria provided are based on the general premise of providing shoulder widths suitable for the traffic demand and adjoining lane configuration. In establishing design guidelines, 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. Care must be exercised in determining shoulder 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. Additional attention is critical when project designs include bridges. Once the shoulder widths are established, there can be a substantial re-design cost associated with even a minor shoulder width adjustment. Rural Highways Shoulder widths are best viewed, as a composite of the overall paved surface needs. For example, shoulder widening is sometimes needed on curves for vehicle off tracking concerns. Rumble strip widths also require additional paved usable shoulder width to accommodate bicycles and pedestrians. In areas where bicycle and pedestrian traffic is common or likely along shoulders, additional shoulder width above the minimum is typical. Additional design guidance is found in the AASHTO Guide for the Development of Bicycle Facilities, MnDOT Bikeway Facility Design Manual and in the MnDOT Road Design Manual.
Page 6 The crash effects of shoulder widths and type as provided in Sections 13.4.2.4 and 13.4.2.5 of the AASHTO Highway Safety Manual (HSM) may be useful in assessing situational benefits or comparing design alternatives. 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. Shoulder width is particularly important on multi-lane streets, where even small variations in design values are multiplied across the cross section. Narrower cross sections reduce crossing distances and have been associated with reduced travel speed; both are direct factors in pedestrian safety. For these reasons, lane and shoulder widths on urban and suburban streets should be designed no wider than to adequately accommodate the vehicular traffic volume and composition. The width of an urban collector street should be planned as the sum of the widths of the ultimate number of lanes for moving traffic, parking, and bicycles, including median width where appropriate. General design guidance is as follows: Specific Design Considerations The following factors should be taken into consideration when selecting shoulder width values as presented previously in Tables 1, 2, and 3, or when determining that a non-standard value may be in order. The design judgment must take into account on all applicable factors rather than only one or two factors weighted inequitably. 1. Average daily traffic (ADT) Shoulder widths for collectors (rural and urban/suburban) are dependent on traffic demand as well as parking and other considerations as listed in Table 1. Rural arterials are also dependent on traffic demand as listed in Table 2. 2. Trucks and large vehicles Heavily traveled, high-speed highways, and highways carrying large numbers of trucks should have usable shoulders of at least 10 feet wide. Where truck traffic exceeds 250 DDHV, a paved (right) shoulder width of 12 feet is required on Interstates, and should be considered for Freeways. Where truck traffic exceeds 250 DDHV (on sections with six or more lanes), a paved (left) shoulder width of 12 feet is required on Interstates, and should be considered for Freeways (must have 12 feet minimum of usable shoulder). Interregional Corridors and Supplemental Freight Routes (should be taken into consideration due to the high volume of truck traffic, as well as highways designated as Oversize/Overweight (OSOW) Super Load Corridors for their unique geometric needs. 3. Bus Shoulder Use Public transit buses are permitted to travel on designated shoulders of congested roadways during peak periods. To qualify for bus shoulder use, existing roadways shall have a minimum roadway shoulder width of 10 feet, and a minimum bridge shoulder width of 11.5 feet. In areas of new construction or reconstruction where bus shoulder use is expected, 12 foot roadway shoulders and 12 foot bridge shoulders shall be constructed. 4. Horizontal alignment elements
Page 7 Shoulder width can play an important design role in stopping and decision sight distance on horizontal curves. Shoulder widening (paved, surfaced or usable) may be necessary to accommodate adequate sight distance. 5. Corridor consistency Shoulders should be constructed to a uniform width for relatively long stretches of roadway. Changes in shoulder width can be used to provide user feedback to a roadway transitional change, such as rural to urban/suburban or in situations of posted (and desired) speed changes. 6. Interrelationships with other cross sectional elements 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 roadbed. 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 shoulder 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. 8. Multimodal considerations All travel modes require efficient, safe, and connected transportation networks. Like motorized vehicles, the ability of non-motorized users to travel along and across streets, highways, and barriers safely and comfortably is a principal measure of livability. To ensure multi-modal network travel, it is necessary to comprehensively analyze a project corridor, as well as adjacent parallel and connecting corridors, to determine how to best create modal balance. After a thorough analysis, designers can determine what facilities may contribute to a complete multi-modal network and how to collaboratively and comprehensively design and implement the network. Where there are no adjacent sidewalks and pedestrians are required to travel on shoulders, the minimum smooth, paved shoulder width is 4 feet. For additional information and guidance regarding pedestrian accessible routes, see the MnDOT Road Design Manual. Bicycle use should be expected on all roads, unless prohibited by law. To accommodate bicycles and pedestrians outside the travel lane, a minimum smooth, rideable, paved area of 4-feet must be provided. Refer to the AASHTO Guide for the Development of Bicycle Facilities, the MnDOT Bikeway Facility Design Manual, the MnDOT Road Design Manual, the rumble strips section within this tech memo, and Tech Memo 11-02-T-02 for additional information. Design features for bicycle usage include shoulders, wide outside lanes, bike lanes, and shared use side paths. When sufficient width is available to provide paved shoulders or bike lanes, they are the preferred
Page 8 facilities on arterial and collector roadways. Refer to the AASHTO Guide for the Development of Bicycle Facilities, the MnDOT Bikeway Facility Design Manual, and the MnDOT Road Design Manual, for appropriate facility selection and design guidance. 9. Context MnDOT s overarching design policy is to balance the objectives of mobility and safety 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 substantial impact on the maintainability for any facility. Temporary Traffic Control requirements often dictate the closure of a traffic lane if equipment setup area and offset distances cannot be accommodated on the shoulder or embankment. This can add traffic control labor, equipment, and material cost and time to maintenance operations for the life of the facility. The responsible operations and maintenance organization must be consulted with and have the ability to provide input. This must occur before the finalization of any cross-sectional elements (e.g. lane width, shoulder width, bike lanes, bus shoulders, etc.). If the desired shoulder widths for maintenance operations are wider than called for in Tables 1 through 3, compare the added cost of the wider shoulders to the added cost to maintenance operations and motorist delay costs as well as other benefits that may be derived. When wider shoulders outside of the parameters in Tables 1 through 3, are chosen, justification must be documented in the project design memo. 11. Traffic Barriers The perceived usable shoulder width is less than the constructed shoulder width when vertical features (such as traffic barrier or walls) are at the edge of the shoulder. This is because roadway users tend to shy away from the vertical feature. For this reason, provide 2 feet of additional widening for shy distance when a barrier is to be installed in areas where the shoulder width will be less than 8 feet. This shy distance is not needed when the section of the roadway shoulder is at least 8 feet wide. 12. Shoulder Stability Paved shoulders eliminate the problem of rutting and drop-offs adjacent to the edge of the traveled way, reduce maintenance, and provide lateral support for the traveled through lane. Unpaved portions of the shoulder can undergo consolidation and rutting over time, resulting in unsafe edge drops. Construction of a Safety Edge at the edge of the paved surface significantly reduces the potential of tire scrubbing, minimizing the consequences of drifting off the pavement surface. On Oversize, Overweight Super Load Corridors, it is preferred that shoulders be paved to the maximum practical (standard) width in order to preserve safety and prevent rutting. 13. Shoulder Contrast It is desirable (but not required) that the color and texture of shoulders be different from those of the traveled way. This contrast helps define the traveled way, especially at night and during inclement weather conditions. Edge line striping is used extensively as a viable alternative to providing contrast. Edge line striping should be applied where bicycle traffic is expected.
Page 9 14. Rumble Strips Shoulder widths that provide less than 4 feet of clear paved-space beyond rumble strips are not adequate to accommodate bicyclists or pedestrians when the shoulder functions as a pedestrian route. Where practical and feasible, Districts are encouraged to provide a minimum of a 6 foot paved shoulder where shoulder rumble strips will be placed on trunk highways with existing or potentially significant bicycle travel. 15. Turnouts Turnouts should be provided where full shoulder widths are not attainable, or economically practical, including roadway alignments that pass through significant rock cuts, or where other unusual environmental conditions exist. The design of turnouts should include significant transition length (for both entry and exit), sufficient width, and adequate sight distance for approaching roadway users. 16. Drainage All shoulders should be sloped to drain away from the traveled way on divided highways with a raised or depressed median. With a raised narrow median, the median shoulders may slope in the same direction as the traveled way, however attention must be given to winter snow melting and refreezing impacts. Surfaces at drainage grates should be smooth/flush with the surface to avoid abrupt edges for bicyclists, pedestrians, and to be ADA compliant. 17. Curbs Vertical curbs should not be used along freeways or other high-speed arterials. Curbs, if needed, along high speed roads, should be of the sloping type (MnDOT D or S) and should not be located closer to the traveled way than the outer edge of the shoulder. Questions When using curbs in conjunction with traffic barriers, consideration should be given to the type and height of barrier. Curbs placed in front of traffic barriers can result in unpredictable and unstable impact trajectories. For more information on curb usage and location near longitudinal barriers and crash cushions, refer to the AASHTO Roadside Design Guide, and the MnDOT Road Design Manual. 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) 366-4623 Jim Rosenow, P.E., Design Flexibility Engineer, MnDOT, at (651) 366-4673 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 http://techmemos.dot.state.mn.us/techmemo.aspx. To add, remove or change your name on the Technical Memoranda mailing list, please visit the web page http://techmemos.dot.state.mn.us/subscribe.aspx -END-