Mitigation Strategies For Design Exceptions - Safety (2024)

Shoulder Width

Shoulders provide a number of important functions. Safety and efficient traffic operations can be adversely affected if any of the following functions are compromised:

• Shoulders provide space for emergency storage of disabled vehicles (Figure 7). Particularly on high-speed, high-volume highways such as urban freeways, the ability to move a disabled vehicle off the travel lanes reduces the risk of rear-end crashes and can prevent a lane from being closed, which can cause severe congestion and safety problems on these facilities.
• Shoulders provide space for enforcement activities (Figure 7). This is particularly important for the outside (right) shoulder because law enforcement personnel prefer to conduct enforcement activities in this location. Shoulder widths of approximately 8 feet or greater are normally required for this function.
• Shoulders provide space for maintenance activities (Figure 7). If routine maintenance work can be conducted without closing a travel lane, both safety and operations will be improved. Shoulder widths of approximately 8 feet or greater are normally required for this function. In northern regions, shoulders also provide space for storing snow that has been cleared from the travel lanes.
• Shoulders provide an area for drivers to maneuver to avoid crashes (Figure 7). This is particularly important on high-speed, high-volume highways or at locations where there is limited stopping sight distance. Shoulder widths of approximately 8 feet or greater are normally required for this function.
• Shoulders improve bicycle accommodation (Figure 8). For most highways, cyclists are legally allowed to ride on the travel lanes. A paved or partially paved shoulder offers cyclists an alternative to ride with some separation from vehicular traffic. This type of shoulder can also reduce risky passing maneuvers by drivers.
• Shoulders increase safety by providing a stable, clear recovery area for drivers who have left the travel lane. If a driver inadvertently leaves the lane or is attempting to avoid a crash or an object in the lane ahead, a firm, stable shoulder greatly increases the chance of safe recovery. However, areas with pavement edge drop-offs can be a significant safety risk. Edge drop-offs (Figure 9) occur where gravel or earth material is adjacent to the paved lane or shoulder. This material can settle or erode at the pavement edge, creating a drop-off that can make it difficult for a driver to safely recover after driving off the paved portion of the roadway. The drop-off can contribute to a loss of control as the driver tries to bring the vehicle back onto the roadway, especially if the driver does not reduce speed before attempting to recover.
• Shoulders improve stopping sight distance at horizontal curves by providing an offset to objects such as barrier and bridge piers (Figure 10).
• On highways with curb and enclosed drainage systems, shoulders store and carry water during storms, preventing water from spreading onto the travel lanes.
• On high-speed roadways, shoulders improve capacity by increasing driver comfort.

Figure 7 is a photo of an urban freeway with three lanes in each direction and shoulders on both the outside and median-side that are 10 to 12 feet wide.

Figure 8 is a photo of a rural two-lane highway with 4-foot paved shoulders and 6-foot granular shoulders.

Figure 9 is a photo showing a dropoff at the edge of the roadway pavement, adjacent to an earth shoulder. A tape measure next to the dropoff shows a 4- to 5-inch difference in elevation.

Figure 10 is a set of two photos. In both photos, vehicles are shown traveling away from the viewer on a highway that curves to the right. A concrete bridge rail is visible to the right of the shoulder in each photo. In the top photo, the shoulder width is narrower, and the bridge rail obscures the view ahead sooner than on the roadway in the lower photo.

Table 7 summarizes the range of minimum shoulder widths for travel lanes and ramps.

Source: A Policy on Geometric Design of Highways and Streets, AASHTO

Clarification: Usable and Paved Shoulders

Design values in the adopted criteria refer to both usable and paved shoulders. A usable shoulder width is the actual width available for the driver to make an emergency or parking stop. This is measured from the edge of traveled way to the point of intersection of the shoulder slope and mild slope (for example, 1:4 or flatter) or to beginning of rounding to slopes steeper than 1:4.

Usable shoulders do not have to be paved. The adopted criteria note that rural arterial shoulders should be paved. FHWA policy does not require a design exception for shoulder type, but rather for the usable shoulder width dimension only.

Clarification: Minimum Shoulder Widths for Interstate Highways

One clarification for shoulder width design exceptions relates to the requirements for Interstates with six or more lanes. The adopted criteria for Interstates specify that the paved width of the right shoulder shall not be less than 10 feet (3.0 meters). Where truck traffic exceeds 250 DDHV (the design hourly volume for one direction), a paved shoulder width of 12 feet (3.6 meters) should be considered. On a four-lane section, the paved width of the left shoulder shall be at least 4 feet (1.2 meters). On sections with six or more lanes, a 10-foot (3.0-meter) paved width for the left shoulder should be provided. Where truck traffic exceeds 250 DDHV, a paved width of 12 feet (3.6 meters) should be considered.

Regardless of the differences in language used in the adopted criteria ("shall," "should be considered," etc.) all of the shoulder widths described above have become standards for the Interstate System by virtue of their adoption by FHWA, and they are the minimum values for each condition described. Therefore, a project designed for the Interstate System that does not provide the applicable shoulder widths would require a formal design exception.

In addition, the incorporation of high occupancy vehicle (HOV) lanes is now common practice on many urban freeways. Lower-cost design solutions have in many cases resulted in the conversion of an existing full-width (12-foot) shoulder to a designated HOV lane. Where conversion of a shoulder to HOV use is being considered and replacement or construction of a new shoulder is not proposed, a design exception is required (potentially for both shoulder width and lateral offset to obstruction).

Substantive Safety

Figure 11 illustrates how variations in shoulder width can affect safety on rural two-lane highways. Note that the substantive safety effects of incremental shoulder widths are less on multilane arterials and on lower-speed urban arterials.

Figure 11 is a graph. The "x" axis is labeled "Average Daily Traffic Volume (veh/day)," and is marked in increments of 500; 1,000; 1,500; 2,000; and 2,500. The "y" axis is "labeled Accident Modification Factor," and is marked in decimal increments of 0.80, 0.90, etc., through 1.60. A note at the top of the "x" axis states, "This factor applies to single–vehicle run-off-road, multiple-vehicle same direction sideswipe accidents, and multiple-vehicle opposite-direction accidents." A horizontal line representing a 6-foot shoulder is the baseline value with an AMF of 1.00. The accident modification factors for the other shoulder widths begin as horizontal lines showing a very minor difference in crash risk at very low traffic volumes. As traffic exceeds 300 vpd, the AMFs increase or decrease linearly and at 2000 vpd, the AMFs return to horizontal lines. At this point the AMF for 8-foot shoulders is 0.87, for 4-foot shoulders is 1.15, for 2-foot shoulders is 1.30, and for 0-foot shoulders is 1.50.

Traffic Operations

Shoulder width has a measurable effect on traffic operations and highway capacity, particularly for high-speed roadways. The interaction of shoulder width with other geometric elements, primarily lane width, also affects operations.

When determining highway capacity, adjustments are made to reflect the effect of shoulder width on free-flow speeds. Table 5 summarizes these effects for rural two-lane highways and Table 8 summarizes effects for freeways.

Source: Highway Capacity Manual

Summary

Table 9 summarizes the potential adverse impacts to safety and operations of a design exception for shoulder width.

Freeway: high-speed, multi-lane divided highway with interchange access only (rural or urban).
Expressway: high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban).
Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, or local).
Urban Arterial: urban arterials with speeds 45 mi/h (70 km/h) or less.

Shoulder Width Resources

• A Policy on Design Standards Interstate System, AASHTO, 2005.
• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004.
• A Guide for Achieving Flexibility in Highway Design, AASHTO, 2004.
• A Guide for Addressing Head-On Collisions, NCHRP Report 500, Volume 4, Transportation Research Board, 2003.
• A Guide for Addressing Run-Off-Road Collisions, NCHRP Report 500, Volume 6, Transportation Research Board, 2003.
• Roadside Design Guide, AASHTO, 2002.
• Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT ≤ 400), AASHTO, 2001.
• Highway Capacity Manual, Transportation Research Board, 2000.
• Guide for the Development of Bicycle Facilities, AASHTO, 1999.
• Highway Safety Design and Operations Guide, AASHTO, 1997.
• Use of Shoulders and Narrow Lanes to Increase Freeway Capacity, NCHRP Report 369, Transportation Research Board, 1995.
• Roadway Widths for Low-Traffic Volume Roads, NCHRP Report 362, Transportation Research Board, 1994.
• FHWA Roadside Hardware Web site http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/

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