Operational Characteristics of 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. High proportions of heavy vehicles compound the problem and contribute to a decrease in safety as impatient drivers attempt to pass slower vehicles in no-passing zones or to pass trucks, despite 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 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 5,000 vehicles per day. A current project expands on that research to develop design guidelines for passing lanes on two-lane highways with higher volumes. The project investigates the effects of volume, terrain, and heavy vehicles on traffic flow. This paper discusses findings from field observations 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 do provide added benefit at higher traffic volumes by reducing delay and percent time spent following. The incremental benefit of adding passing lanes or adding length to lanes diminished, and the effects of ADT on operations were more substantial than the effects of terrain or truck percentage for the study corridor.