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.