With more than 57,000 miles of two-lane rural highways in Texas — where it’s estimated that 68 percent of rural travel occurs — Texas drivers are known to pull onto the shoulder to let a vehicle pass, sometimes creating an unsafe situation. Developing and increasing use of low-cost roadway safety and capacity improvements are key to stretching available funding for rural highways, while also promoting a safer rural driving experience. One such improvement heavily used in Texas, known as the Super 2 design, is a periodic, short-term passing lane — either alternating or side by side — at regular intervals along a rural, two-lane highway corridor.
Since 2001 and following research from Texas A&M Transportation Institute (TTI) projects, the Texas Department of Transportation’s (TxDOT’s) use of Super 2s has steadily increased, with other states following Texas’ lead. From the start, continued TTI research and assistance with implementation of Super 2s have helped Texans, providing operational and safety benefits at much lower costs than traditional approaches.
Design Guidelines for Super 2s
The initial 2001 TxDOT project (0-4064) developed reliable, defensible guidelines and criteria for TxDOT’s Roadway Design Manual on use of Super 2s. Researchers confirmed that the Super 2 concept added little additional cost to rehabilitation projects, with little or no right-of-way adjustment. The guidelines addressed optimum passing lane length and spacing, shoulder width requirements, and signage and pavement marking strategies. Benefit/cost analyses done within TxDOT at the time, along with implementation predictions at the close of the project, indicated the improved design criteria and guidelines would save the state millions of dollars — even with a conservative estimate of one additional Super 2 section every three years.
Super 2 Design for Higher Traffic Volumes
A 2011 effort (TxDOT Project 0-6135) expanded on findings from 2001 to describe operational and safety benefits of Super 2s for two-lane roads with traffic volumes between 5,000 and 15,000 vehicles per day. Results showed reductions in delay and percent time following for a variety of combinations of volume, terrain and up to 20 percent heavy vehicles. Analysis of crash data revealed a 35 percent reduction in expected non-intersection injury crashes and a 42 percent reduction for segments with intersections.
“We used the results from this project to produce revisions to TxDOT’s Roadway Design Manual that allow for use of Super 2s with these higher volumes in certain instances,” says TTI Research Engineer Marcus Brewer, principal investigator on the project. “But it’s also important that we keep analyzing how long these lanes should and shouldn’t be, and when adding capacity is no longer effective.”
Developing Capacity and Cost Benefits of Super 2 Corridors
TTI recently began the latest project in this series of Super 2–related research efforts for TxDOT (Project 0-6997). “Due to their history of proven success in reducing delay and crashes on two-lane roads, there continues to be increased interest in Super 2s as a solution,” says Mark Wooldridge, director of maintenance in the TxDOT Houston District. Wooldridge also served on the previous studies as a researcher and a project director.
In fact, use of the Super 2 design is one option for dealing with the energy-sector traffic congestion increasing from heavy-truck traffic in those affected parts of the state, such as the Permian Basin. (In related efforts, for example, TTI Agency Deputy Director Bill Stockton and other researchers are supporting recent Institute research into the effects of heavy-truck traffic on rural highways as a result of increased oil development in West Texas.) But specific benefits in improving capacity related to reductions in percent time following are still not known. In addition, as more passing lane length is added to a Super 2 corridor, the more it may resemble a traditional four-lane alignment, reducing the unique benefits of a Super 2 treatment.
The current project will look at when it becomes beneficial to add passing lane length and/or additional passing lanes along a Super 2 corridor, as well as the comparative costs of Super 2 corridors and traditional two-lane and four-lane alignments. Researchers will then develop a tool that will help TxDOT determine the most appropriate locations to build or extend Super 2 corridors and to prioritize upcoming projects.
According to Wooldridge, TxDOT wants to ensure that Super 2s continue to be the right solution in the right places — often on roads where no other options exist for higher-cost safety and operational improvements.
“Continued research to develop design and planning tools that assist in those decisions is an important piece in the puzzle of figuring out which improvement is best for which road,” Wooldridge says.
