Safety Evaluation of Buffer-Separated High-Occupancy Vehicle Lanes in Texas

Abstract

In Texas, high-occupancy vehicle (HOV) lanes are an integral part of urban mobility. Although an extensive system of permanent HOV lanes is planned for the Dallas–Fort Worth area, the Texas Department of Transportation and Dallas Area Rapid Transit have implemented interim HOV lanes by retrofitting them into existing freeways. Safety is examined for Dallas's buffer-separated concurrent-flow HOV lanes, which were implemented by lane widths being reduced and by the inside shoulder being converted to an HOV lane on I-35 East and I-635. Injury crash data from each corridor were analyzed on the basis of crash rates, frequency trends, and manually reviewing police reports. The analysis considered the impact of design elements, including buffer width, shoulder presence, and lane width. Operationally, the analysis considered the impact of speed differential between the HOV and general purpose lanes. This evaluation resulted in three key findings: (a) both corridors had an increase in crash rates after implementation of the HOV lane, (b) the increase in crashes is primarily focused on the HOV lane and the first adjacent general purpose lane, and (c) the increase in crashes is primarily attributed to the speed differential between the HOV and the general purpose lanes and the reduced HOV cross section. The recommendation, based on these findings, is to provide greater width for the total HOV cross section (inside shoulder + HOV lane + painted buffer) than that provided in the two interim corridors. An absolute minimum of 18 ft between the freeway barrier and the general purpose lanes may mitigate many types of crashes that occur because of the speed differential, with full inside shoulders being the desirable cross section.