Evaluation of the Virginia Department of Transportation Adaptive Signal Control Technology Pilot Project

About the report:

Currently, most traffic signals operated by the Virginia Department of Transportation (VDOT) use actuated plans that vary by time of day (TOD) and day of the week. These timing plans are typically developed off-line using traffic count information collected in the field and then processed using signal optimization software. This method works well as long as traffic volumes remain consistent with the conditions used to develop the timing plan, but timing plans can become suboptimal if traffic demands deviate from those conditions. Traffic growth over time, seasonal changes in traffic, special events, or incidents can all cause TOD plans to perform poorly, resulting in increased delays to drivers. As a result, VDOT must regularly retime signalized intersections to deal with long-term changes in travel patterns, which incurs costs to VDOT. Even so, non-recurring events can still cause TOD plans to perform poorly.

Adaptive signal control technology (ASCT) is one tool that has been proposed to handle variable traffic demand better. VDOT’s Traffic Engineering Division began a pilot program to install the InSync ASCT developed by Rhythm Engineering on 13 corridors around the state beginning in 2011. The InSync system uses enhanced detection along a corridor to adjust signal timing parameters dynamically to meet observed demand in real time, eliminating the need to develop static timing plans. This allows the ASCT system to adjust signal timing parameters to account for variations in flow attributable to special events, seasonal flows, incidents, or simply the increase of volumes over time. In this case, signal timings are not pre-defined based on historic data, so ASCT systems can potentially reduce delays created by outdated static TOD plans.

These pilot deployments were evaluated to determine if ASCT created operational and safety improvements large enough to justify the additional costs to install ASCT. Data on mainline traffic operations, side street delays, and intersection crashes were collected with and without ASCT active. The results showed that mainline traffic operations generally improved if (1) the corridor was not oversaturated; (2) the corridor did not have characteristics that encourage platoon dispersion; and (3) the corridor did not already function well. Side street delays generally increased, although net benefits in overall corridor travel time were usually still observed. An empirical Bayes safety analysis of crashes at the intersections where ASCT was installed also found a 17% decrease in total crashes. Overall, ASCT generally produced a favorable benefit/cost ratio. The findings from the pilot tests were used to identify key considerations for future ASCT deployments so that VDOT could better identify future sites that might benefit from ASCT installation.

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