Study Details

Study Title: Safety Performance Evaluation of Urban Undivided Four-Lane to Five-Lane Conversion in Louisiana

Authors: Rahman et al.

Publication Date: 2018

Abstract: This paper compares the safety performance of urban five-lane highways with a two-way left turn lane (5T) with four-lane undivided highways (4U). Road diet (three-lane highway with a two-way left turn lane) remains the most prevalent cost-effective alternative to 4U, still some 4U highway segments have been converted to 5T with an aim of coping with the increasing left-turn traffic towards the roadside establishments without sacrificing the capacity. This study provides an extensive look at the safety performance of 4U to 5T conversion. A total of nine converted sites of different characteristics were evaluated using up to seven years of crash data before and after conversion. The Empirical Bayes (EB) method was used with LaDOTD developed Safety Performance Function (SPF) and estimated temporal factors to avoid any potential regression-to-the-mean bias. The reduction of crashes was achieved in all the sites with an estimated overall CMF of 0.482 with a small standard deviation of 0.001, indicating a total crash reduction of 51.8% after conversion. The relationship of CMF with driveway density showed that 4U to 5T conversion performed better in driveway densities up to 55 driveways per mile. Analysis by crash type shows substantial reduction of the predominant rear-end crashes on converted urban 5T highways. The high benefit-cost ratio, 77, indicates that 4U to 5T conversion proves to be an effective low-cost crash countermeasure for urban and suburban roadways with low to moderate AADT.

Study Citation: Rahman, M.A., X. Sun, and S. Das. "Safety Performance Evaluation of Urban Undivided Four-Lane to Five-Lane Conversion in Louisiana". Presented at the 97th Annual Meeting of the Transportation Research Board, Paper No. 18-06321, Washington, D.C., (2018).

Study Report: Download the Study Report Document


CMFs Associated With This Study

Category: Roadway

Countermeasure: Four to five lane conversion

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.48251.83 StarsNon-intersectionAllNot specifiedUrban
0.35564.53 StarsNon-intersectionAllNot specifiedUrban and suburban
0.56443.63 StarsNon-intersectionAllNot specifiedUrban and suburban
0.65834.23 StarsNon-intersectionAllNot specifiedUrban and suburban
0.21793 StarsNon-intersectionAllNot specifiedUrban and suburban
0.42657.43 StarsNon-intersectionAllNot specifiedUrban and suburban
CMF Equation3 StarsNon-intersectionAllNot specifiedUrban
CMF Equation3 StarsNon-intersectionAllNot specifiedUrban