Issues Affecting Performance of Adaptive Traffic Control Systems in Oversaturated Conditions

This paper identifies three broad issues that may negatively affect the performance of an adaptive traffic control system operating a coordinated arterial when oversaturated conditions arise on at least one intersection approach. The first issue is underallocating green time to the critical (oversaturated) approach at the bottleneck intersection. Second, more green time than necessary may be given to the peak (critical) direction at intersections upstream of the bottleneck, and offsets that favor progression in the critical direction may be used inappropriately when the active bottleneck counters any benefits associated with green waves in the peak direction. Finally, offsets at intersections downstream of the bottleneck may be set inefficiently and result in additional delays for traffic departing the bottleneck and creating the potential for queue spillbacks to the bottleneck itself. Possible remedial actions involve reallocating green time to the critical approach until residual queues form on conflicting approaches (Issue 1), reducing green time at signals upstream of the bottleneck in the peak direction so that signal capacities are equivalent to the bottleneck capacity (Issue 2), and manually designing an offset plan that gives priority to traffic departing the bottleneck and protects the traffic's full green bandwidth (Issue 3). These problems are validated and analyzed on a 5-mi adaptively controlled segment of Pacific Coast Highway in Los Angeles, California, with field data from March through June 2013. The Los Angeles adaptive traffic control system is used for the validation and analysis.