Operations and Safety of Super 2 Corridors with Higher Volumes

As traffic volumes increase, 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. Previous research in Texas demonstrated that periodic passing lanes can improve operations on two-lane highways with average daily traffic (ADT) lower than 5000; these "Super 2" highways can provide many of the benefits of a four-lane alignment at a lower cost. This project expands on that research to develop design guidelines for passing lanes on two-lane highways with higher volumes, investigating the effects of volume, terrain, and heavy vehicles on traffic flow and safety. This report 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, reducing crashes, delay, and percent time spent following. Empirical Bayes analysis of crash data reveals a 35 percent reduction in expected nonintersection injury crashes. Simulation results indicate that most passing activity takes place within the first mile of the passing lane, so providing additional passing lanes can offer greater benefit than providing longer passing lanes. Whether adding new passing lanes or adding length to existing lanes, the incremental benefit diminishes as additional length is provided and the highway more closely resembles a four-lane alignment. The simulation study also showed that the effects of ADT on operations were more substantial than the effects of terrain or truck percentage for the study corridor.