Operational and Safety Trade-Offs: Reducing Freeway Lane and Shoulder Width to Permit an Additional Lane

Abstract

This research effort identified the operational and safety implications of the use of reduced lane and shoulder widths for a variety of urban freeway configurations. The study used speed, crash, and geometric data for freeways in Dallas, Houston, and San Antonio, Texas. The operational analysis identified an increase of about 2.2 mph in speed for a 12-ft lane compared with 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. These findings suggest that the width of the left shoulder is more important with a reduced lane width. Operating speeds on Texas freeways are 2 mph lower during nighttime (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 freeways with 12- to 11-ft lanes were compared. There was a reduction in fatal and injury crashes that ranged from 5% for two-lane freeways up to 12% for five-lane freeways. Similarly, some crash reductions were associated with each additional lane, increased left shoulder widths, and increased right shoulder widths. This paper concludes with a series of sample problem scenarios that collectively demonstrate how the resulting operating speed and crash prediction models allow practitioners to assess the trade-offs that can be expected when urban freeway lane and shoulder widths are modified.