Super 2 Highways in Texas: Operational and Safety Characteristics

As traffic volumes increase in both urban and rural areas, so do demands on the highway network. Specifically, as rural traffic volumes rise in Texas, the pressure on the state's network of two-lane highways rises accordingly. Previous research in Texas demonstrated that periodic passing lanes can improve operations on two-lane highways with average daily traffic lower than 5,000 vehicles. These highways, called Super 2 highways, can provide many of the benefits of a four-lane alignment at a lower cost. A recent project expanded on that research to develop design guidelines for passing lanes on two-lane highways with higher volumes. Researchers investigated the effects of volume, terrain, and heavy vehicles on traffic flow and safety. This paper discusses findings from field observations and crash analysis of existing Super 2 highway corridors in Texas and computer modeling of traffic conditions on a simulated Super 2 corridor. Results indicate that passing lanes provide added benefit at higher traffic volumes by reducing crashes, delay, and percent time spent following. Empirical Bayes analysis of crash data reveals a 35% reduction in expected nonintersection crashes with injuries. Simulation results indicate that most passing activity takes place within the first mile of the passing lane, so additional passing lanes can offer greater benefit than longer passing lanes. Whether new passing lanes are added or existing lanes are lengthened, the incremental benefit diminishes as additional length is provided and the highway more closely resembles a four-lane alignment.