Implementation of an adaptive transit signal priority system resulted in 5 percent reductions in running time and 18 to 32 percent reductions in bus intersection delays in San Mateo County.

Results of a field operational test in San Mateo County, California.

June 2010
San Mateo County,California,United States

Summary Information

Many existing transit signal priority (TSP) systems do not take into account the real-time location and other available travel statistics, such as bus speed, when choosing to grant priority to buses. Most systems rely on location-based detection, which provides "short notice" to the signal, which can generate undesirable delays for several phases after the priority is granted, particularly in the cross-street directions. However, most bus systems in the US now are equipped with automatic vehicle location (AVL) technology, which allows for improved TSP. In light of the above considerations, California PATH, in collaboration with Caltrans Headquarters and District 4 has been developing an Adaptive Transit Signal Priority (ATSP) concept. The main features of the ATSP concept include:
  • Providing priority to transit vehicles if warranted by accepted criteria. This attempts to provide a trade-off between bus delay savings and the impacts on the rest of the traffic.
  • Making real time decisions adaptive to the movements of transit vehicles, traffic conditions and signal status. Again, this reflects the desire to balance the criteria of signal priority with competing interests at the intersection.
  • Utilizing the existing GPS/AVL systems installed on buses to continuously monitor bus locations and predict bus arrival times at intersections. This builds functionality on existing technology in a cost-effective manner.
  • Building the system using a wide range of closed-loop signal control systems, including 170E controllers. This may have potential for wide-scale implementation given an existing installed technology base.
The ATSP concept was tested on San Mateo Transit (SamTrans) buses on routes 390 and 391, which travel along the 6.6 mile test section of El Camino Real that includes 35 signalized intersections. Fifteen SamTrans buses were outfitted to utilize the ATSP system along those routes.

The ATSP system prioritizes the type of signal priority based on the data collected from the AVL system, which determines if priority is necessary, and if so, if elongating the green will be sufficient. If the vehicle is unlikely to make the signal with an elongated green phase, the system will request an early green for the next phase, shortening the green phase of the cross-traffic, which can be disruptive to the system. The ATSP was integrated with a cell phone based emulation of the ACS since this was only a test project and integration with the actual ACS system was not feasible. The field operational test demonstrated that the existing capacity in the communications system is sufficient for the integration of the ATSP into the AVL system, with very few polling requests dropped by the system.

The evaluation was conducted as a comparison between two, six week "before" and "after" periods for the equipped vehicles.
  • Average bus running time was reduced by 4.9% (northbound) and 5.6% (southbound).
  • Bus delays at intersections was reduced by 18.5% for northbound trips and 32.0% for southbound trips.
  • Number of stops for red was reduced by 9.5% (northbound) and 16.8% (southbound).
  • Average waiting time per stop for red was reduced by 10.5% (northbound) and 18.5% (southbound).
  • All of the above changes are statistically significant at the 5 percent significance level.
  • ATSP has no statistically significant impacts on traffic intersection delays.


    For more information on the utilization of excess communications capacity and dynamic passenger information applications, see [2013-00818].

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Field Operational Tests of Adaptive Transit Signal Priority Systems

Author: Meng Li, Kun Zhou, Wei‐Bin Zhang, Yue Li, Guoyuan Wu, Fanping Bu, Sonja Sun, Kai Leung, and James Lau

Source Date: June 2010



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Goal Areas


Typical Deployment Locations

Metropolitan Areas


bus priority, traffic signals, TSP, automated vehicle location, computer aided dispatch, automatic vehicle locator, AVL, CAD, AVL/CAD, adaptive transit signal priority, AVL, ACS, automatic vehicle location, advanced communication system

Benefit ID: 2013-00819