Where a “full width” shoulder cannot be achieved, the designer should strive to provide as wide a shoulder as possible that meets functional requirements. A major function of the shoulder is to act as part of the clear zone. Mitigating a narrow shoulder can include the provision of an extra-wide clear zone or milder side-slope to partially counteract the loss of the shoulder. The use of traversable ditch designs may also be appropriate where narrow shoulders are used. Where a “full width” shoulder cannot be achieved, the designer should strive to provide as wide a shoulder as possible that meets functional requirements.
NCHRP Report 254: Table 5: Shoulder Geometrics and Use Guidelines shows functional requirements for a range of shoulder widths. A major function of the shoulder is to act as part of the clear zone. Mitigating a narrow shoulder can include the provision of an extra-wide clear zone or milder sideslope to partially counteract the loss of the shoulder. The use of traversable ditch designs may also be appropriate where narrow shoulders are used.
Another important function is the storage of disabled or stopped vehicles. If a full, continuous shoulder is not possible, designers should at least seek to provide intermittent full-width turnouts, especially on higher-volume, high-speed roads. Vehicles involved in a crash that still occupy all or part of a travel lane are major causes of congestion, which in turn can create safety problems upstream of the incident. The provision for full or at least “operationally functional” shoulder width associated with vehicle refuge and law enforcement supports incident management.
In urban and some suburban areas, curbs are often used adjacent to the travel lanes where shoulders are not provided. When faced with topographical and/or environmental constraints, the designer may use curbing at the outside edge of the shoulder along with an enclosed drainage system. Curbing is primarily used to direct storm water flow to enclosed drainage systems and to control access. Curbing may also be used for delineation and aesthetics. In these situations, appropriate transitions should be provided between the shoulder and the curbed cross section. Curbing has little, if any, redirective capacity at medium and high speeds and can destabilize vehicles or cause them to vault traffic barriers. The designer should be aware that curbs are not adequate to prevent a vehicle from leaving the roadway, and the curb provides a minimally redirective function only for vehicles traveling at very low speeds and low impact angles. Curbing should, therefore, be generally used only for access control, drainage, or erosion control. When curbing is needed for drainage control on moderate- or high-speed facilities, sloping curbs should be used, placed as far away as practical from the traveled way to minimize the destabilizing effects on vehicles that might strike it.
Source: A Guide for Achieving Flexibility in Highway Design
Published: 2004
From A Guide for Achieving Flexibility in Highway Design, 2004, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission. AASHTO publications may be purchased from that organization’s bookstore at 1-800-231-3475 or online at https://bookstore.transportation.org/.
