Reducing Lane and Shoulder Width to Permit an Additional Lane on a Freeway

This research effort identified the operational and safety implications of using reduced lane and shoulder widths for a variety of freeway configurations. The research team used speed, crash, and geometric data for freeways in Dallas, Houston, and San Antonio. The operational analysis identified an increase of about 2.2 mph in speed for a 12-ft lane as compared to an 11-ft lane. The shoulder width is significant when the adjacent lane is 11-ft wide, but not when it is 12-ft wide which suggests that left shoulder width is more important with a reduced lane width. Operating speeds on Texas freeways are 2 mph lower during night time (with roadside lighting present) than during the day. Speeds were higher (by 1.5 mph) on the weekends (Saturday) than on the weekday studied (Wednesday). The safety analysis determined a crash difference when comparing freeways with 12 ft to 11 ft lanes. There is a reduction in fatal or severe injury (KAB) crashes that ranges from 5% for 2-lane freeways up to 12% for 5-lane freeways, other roadway characteristics equal. Similarly, there are crash reductions associated with each additional lane, increased left shoulder widths, and increased right shoulder widths. While constructing an additional lane is beneficial in terms of safety, a larger safety detriment caused by narrow lanes or shoulders annuls such benefit. However, if it is possible to increase the total paved width when adding a travel lane, the safety model allows the analyst to identify lane and shoulder widths so that the number of crashes along the corridor will expectedly remain unchanged.

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