Traffic signal priority is simply the idea of giving special treatment to transit vehicles at signalized intersections. Since transit vehicles can hold many people, giving priority to transit can potentially increase the person throughput of an intersection.
By contrast, an active priority strategy involves detecting the presence of a transit vehicle and, depending on the system logic and the traffic situation then existing, giving the transit vehicle special treatment. The system can give an early green signal or hold a green signal that is already displaying. An active system must be able to both detect the presence of a bus and predict its arrival time at the intersection. Near-side stops can complicate the prediction of intersection arrival times. Real-time control strategies can consider not only the presence of a bus but the bus adherence to schedule and the volume of other traffic. One common strategy is to give priority only to late buses (compared to the scheduled time) but not to early buses. This strategy optimizes schedule adherence (and therefore waiting time) rather than running time.
There are many different options for signal priority logic. Real-time, adaptive systems can incorporate information on traffic flow, flow coordination, bus schedule adherence, and prior bus arrival times.