Reconfiguring Urban Undivided Four-Lane Highways to Five-Lane: A Nonideal but Very Effective Solution for Crash Reduction

Abstract

Although a reconfiguration to three-lane with a left turn lane remains the most prevalent cost-effective countermeasure, some four-lane undivided highway segments in Louisiana have been reconfigured to five-lane segments with a left-turn lane by utilizing the existing pavement width basically at the cost of restriping. The current study analyzes nine locations where undivided four-lane roadways were converted to five-lane roadways in Louisiana urban areas with up to 7 years of before-and-after crash data. To avoid any potential regression-to-the-mean bias, the empirical Bayes (EB) method was used with the safety performance function (SPF) developed by the Louisiana Department of Transportation and Development (LaDOTD). Despite the contemplation in design guidelines and previous studies that a five-lane roadway is more crash-susceptible than a four-lane roadway, the safety effectiveness of conversion to five-lane was deduced to be significantly positive. Consistent crash reduction was observed in all sites, which resulted in a crash modification factor (CMF) of 0.48 with a small variance of 0.001. Expectedly, a substantial reduction in the target crash type, rear-end crashes, was achieved. This four-lane to five-lane conversion was found very effective specifically for urban and suburban roadways with annual average daily traffic (AADT) ranging from 15,000 to 30,000 and driveway density from 18 to 31 driveways per kilometer (30–50 driveways per mile). The high safety benefit-cost ratio, 78∶1, indicates strong support to use this countermeasure on four-lane undivided roadways with additional evaluation for feasibility.