Passing Behavior of Drivers on Super 2 Highways in Texas

As traffic volumes increase in many jurisdictions, in both urban and rural areas, the demand on the highway network also increases. Specifically, as rural traffic volumes rise in Texas, the pressure on the state’s network of two-lane highways rises accordingly. The increased volumes have an effect on congestion, air quality, and safety as traffic density increases, often approaching the limits of capacity for two-lane highways. High proportions of heavy vehicles compound the problem, contributing to a decrease in safety as impatient drivers attempt to pass slower vehicles in no-passing zones or pass trucks despite having diminished sight distance beyond such vehicles. Previous research in Texas demonstrated that periodic passing lanes can improve operations on two-lane highways with low to moderate volumes; these “Super 2” highways can provide many of the benefits of a four-lane alignment at lower cost. The current Texas Roadway Design Manual contains these guidelines for highways with Average Daily Traffic (ADT) lower than 5000. A current research project is expanding on that research to develop design guidelines for length and spacing of passing lanes on two-lane highways with higher volumes. That project will consider the effects of volume and terrain on traffic flow; in addition, the effects of varying proportions of heavy vehicles will also be considered. This paper will discuss the findings from an observation of passing maneuvers on existing Super 2 highway corridors in Texas, with an emphasis on where passing maneuvers occur within the passing lane and potential implications on congestion, crash reduction, and improved safety.