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What's That Extra Two Feet Of
Lane Or Shoulder Width Worth?

By WILLIAM T. SUNLEY, Engineer of Local Roads and Streets
Illinois Department of Transportation

Percent Accident Reduction For Lane Widening Only
(For existing lane widths between 8 and 12 feet)

Amount of Lane
Widening (Ft.)

Percent Reduction in
Related Accident Types









Percent Accident Reduction
For Shoulder Widening Only

(For existing shoulder widths between 0 and 12 feet)

Amount of Shoulder
Widening per Side (Ft.)

Percent Reduction in
Related Accident Types















There are an estimated 3.1 million miles of rural two-lane highway in the United States, representing 97 percent of the total rural mileage and 80 percent of all highway miles. In the two-lane rural highway system, 59.5 percent has lane widths of 10 feet or less. These narrow lanes do not provide an adequate margin of error for vehicles and therefore indirectly contribute to the cause of many accidents. Faced with upgrading this existing two-lane rural highway system, local highway authorities need to consider the relationship between accidents and various geometric and roadside features. Many studies have been conducted to determine the effect of various combinations of lane width, shoulder width, and shoulder surface type on accident rates. The type of accidents found to be most related to cross-section features include single-vehicle fixed-object collisions, rollover or run-off the road accidents, multi-vehicle head-on collisions, and opposite direction sideswipe or same direction sideswipe. The need exists for cost-effective improvements to roadway and roadside design that will reduce the number of these accidents.

One recent study conducted by the Transportation Research Board, found in Transportation Research Record 1195, collected and analyzed accidents, traffic, roadway, and roadside data from 4900 miles of two-lane roads in seven states covering a wide range of traffic and geometric conditions. An accident predictive model, developed for two-lane rural roads having ADTs between 50 and 10,000, lane widths of 8 to 12 feet, and shoulder widths of zero to 12 feet, was utilized along with detailed statistical procedures to determine expected accident reductions related to geometric improvements. The following two charts show key results from the study.

These two charts illustrate the importance of widening a lane or shoulder an extra 2 feet. The first foot of lane widening (widening lanes from 9 to 10 feet for example) corresponds to a 12 percent reduction in related accidents while 2 feet of widening (from a 9 foot lane to an 11 foot lane) results in a 23 percent reduction. An additional 2 feet of shoulder widening yields slightly smaller reductions in related accidents, although still significant at 16 percent for paved shoulders and 13 percent for unpaved shoulders. So you see, that extra two feet of widening does have some value.

In the recently revised "Administrative Policies" manual issued by the Illinois Department of Transportation's Bureau of Local Roads and Streets, a new "3R

April 1990 / Illinois Municipal Review / Page 19

Policies for Local Agency MFT Improvements" and new policies for local residential streets were added. Realizing that available financing does not always permit the reconstruction of existing highway facilities to ideal standards, these "3R Policies" (Resurfacing, Rehabilitation and Restoration) were formulated to allow local agencies to upgraded serviceability and make appropriate safety improvements with limited funding.

The thrust of these "3R Policies" is to provide (1) wider and smoother riding surfaces, (2) wider and structurally improved bridges, (3) safer roadsides, and (4) improvements to identified high accident locations. These policies were not intended to be used based on economic considerations alone. For specific sites involving high accident locations, care should be taken to eliminate as many of these accidents as possible, either by increased lane or shoulder width or by other safety improvements. By following these policies and realizing the advantage of wider lane and/or shoulder widths, local agencies can provide safer highway systems to the motoring public.

Credits to: Tom Domagalski, Local Policy and Procedure Engineer

Page 20 / Illinois Municipal Review / April 1990

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