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FHWA Home / Safety / Geometric Design / Publications / Mitigation Strategies For Design Exceptions

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:

Figure 7. Shoulders on this urban freeway provide enough width for crash avoidance, storage of disabled vehicles, maintenance activities, and enforcement.

FIGURE 7

Shoulders on this urban freeway provide enough width for crash avoidance, storage of disabled vehicles, maintenance activities, and enforcement.

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.  Partially-paved shoulders on this rural arterial improve bicycle accommodation and reduce risky passing maneuvers.

FIGURE 8

Partially-paved shoulders on this rural arterial improve bicycle accommodation and reduce risky passing maneuvers.

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

Figure 9.  Pavement edge drop-off.

FIGURE 9

Pavement edge drop-off.

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.  Comparison of how shoulder width affects stopping sight distance past concrete bridge rail along horizontal curves.

Figure 10.  Comparison of how shoulder width affects stopping sight distance past concrete bridge rail along horizontal curves.

FIGURE 10

Comparison of how shoulder width affects stopping sight distance past concrete bridge rail along horizontal curves.

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.

TABLE 7

Ranges for Minimum Shoulder Width

Type of Roadway

Rural

Urban

US (feet)

Metric (meters)

US (feet)

Metric (meters)

Freeway

4–12

1.2–3.6

4–12

1.2–3.6

Ramps (1–lane)

1–10

0.3–3.0

1–10

0.3–3.0

Arterial

2–8

0.6–2.4

2–8

0.6–2.4

Collector

2–8

0.6–2.4

2–8

0.6–2.4

Local

2–8

0.6–2.4

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.  Accident Modification Factors for Shoulder Width on Rural Two-Lane Highways. (Source:  Prediction of the Expected Safety Performance of Rural Two–Lane Highways, FHWA)

FIGURE 11

Accident Modification Factors for Shoulder Width on Rural Two-Lane Highways.

(Source:  Prediction of the Expected Safety Performance of Rural Two-Lane Highways, FHWA)

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.

TABLE 8

Operational Effects of Freeway Shoulder Widths

Right-Shoulder Lateral Clearance (ft)

Reduction in Free-Flow Speed (mi/h)

Lanes in One Direction

2

3

4

≥5

≥6

0.0

0.0

0.0

0.0

5

0.6

0.4

0.2

0.1

4

1.2

0.8

0.4

0.2

3

1.8

1.2

0.6

0.3

2

2.4

1.6

0.8

0.4

1

3.0

2.0

1.0

0.5

0

3.6

2.4

1.2

0.6

Right-Shoulder Lateral Clearance (m)

Reduction in Free-Flow Speed (km/h)

Lanes in One Direction

2

3

4

≥5

≥1.8

0.0

0.0

0.0

0.0

1.5

1.0

0.7

0.3

0.2

1.2

1.9

1.3

0.7

0.4

0.9

2.9

1.9

1.0

0.6

0.6

3.9

2.6

1.3

0.8

0.3

4.8

3.2

1.6

1.1

0.0

5.8

3.9

1.9

1.3

Source:  Highway Capacity Manual

Summary

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

TABLE 9

Shoulder Width:  Potential Adverse Impacts to Safety and Operations

Safety & Operational Issues

Freeway

Expressway

Rural
Two-Lane

Urban Arterial

Run-off-road crashes

X

X

X

Assumed cross section with curb and gutter (no shoulders)

Cross-median crashes

X

X

Cross-centerline crashes

X

Pavement edge dropoffs

X

X

X

Rear-end crashes if operations deteriorate (abrupt speed reduction)

X

X

X

Lane blockage from incidents

X

X

X

Reduced free-flow speeds

X

X

X

Shying away from the edge of the roadway

X

X

X

Inadequate space for enforcement activities and emergency response

X

X

X

Inadequate space for emergency pullover

X

X

X

Inadequate space to avoid crashes or objects on the travel lanes

X

X

X

Lack of storage space for disabled vehicles

X

X

X

Bicyclists forced onto the travel lanes.

X

X

X

Inadequate space for maintenance activities

X

X

X

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

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Page last modified on October 15, 2014.
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