**4. Passive versus active**

Passive or inactive TSP refers to an initial method of signal priority which adjusts the signal timing offline while relying on the historical data. This adjustment mainly changes signal time parameters including split length, offset, and cycle length. The objective of signal setting with respect to passive TSP is to increase the probability of transit vehicles arriving at the intersection during the green interval. However, passive TSP is inflexible in adapting to the dynamic flow of traffic and bus conditions. The reason is that passive priority always provides a green light to transit even if there is no transit vehicle; not to mention about the delay it would cause to the other conflicting phases by giving ineffective green to the bus-phases. Passive priority becomes more effective when the traffic volume is light or moderate, with high transit frequencies, and predictable transit travel time [40]. Passive priority is cheap and easy-to-implement; both are advantages, since the transit detection and communication equipment are not required. It is worth noting here that preemption priority applies priority tactics abruptly. This is sometimes done by interrupting signal operation by skipping phases or terminating pedestrian clearance time, in order to permit a specific vehicle (e.g. ambulance) pass through the traffic light. Preemption can be considered as the highest level of priority, which is frequently used for emergency vehicles [41].

Contrary to passive priority, active TSP is about granting priority tactics in real time and only to those transit vehicles that are present or about to approach the signalized intersections. In an active priority system, the real-time information regarding transit vehicles' speed and location should be detected. Some standard vehicle/bus detection techniques are inductive loops, infrared, and radio based systems which are considered as static detection or selective vehicle detection (SVD) [42, 43]. On the other hand, the automatic vehicle location (AVL) system is another transit detection approach that provides dynamic monitoring of transit location. Taking into account the use of detectors, TSP logic is activated when the transit

vehicle passes the check-in detector, which is located upstream of the signal. Where to put the check-in detector is not deterministic and its optimal location is mostly related to traffic demand, and signal timing. The result demonstrated that putting a detector between 450 ft. (150 m) and 900 ft. (300 m) upstream of the intersection can output better results [43]. Meanwhile, the detection should cancel out the priority request when transit passes the stop-line detectors (check-out detectors). Those are located just after the traffic light, indicating the transit vehicle received priority, could pass through, and it is the time to start compensating the amount of time taken from the conflicting phases. Active TSP has been demonstrated as a better approach to improve transit performance, to better accommodate uncertain arrival time, and make on-street transit more reliable, faster, and cost-effective [42, 44]. Active TSP has been taken into consideration worldwide. For instance, applying active signal priority was studied on the two old and large street-car systems in Melbourne, Australia, and Toronto, Canada [45]. The results confirmed that such an approach is a cost-effective approach to manage traffic systems.

Song et al. [46] compared the GPS-based TSP and traditional TSP on two corridors in Utah, and it was found that GPS-based TSP reduced the same delay and travel time similarly to the traditional TSP. Surprisingly, the GPS-based signal priority system was effective in the flexible detection zone and could bring conditional priority into its logic while causing smaller impact on the side-street traffic. Active priority has recently focused its attention not only on the presence of transit vehicles, but rather on applying priority logic based on some conditions.

Unconditional priority means granting TSP tactics to the upcoming transit vehicles regardless of cross-street traffic or queue length, state of signal, or transit arrival time. It is more of an aggressive approach toward granting priority. In other words, unconditional priority is beneficial in improving bus delays, travel time, and reliability when the bus frequency is low, and when the traffic demand over signal is low. On the other hand, conditional priority grants transit signal priority only if the state of signal and bus arrival meet some defined requirements. For instance, conditional TSP can be applied if some of the following criteria are met: transit is behind schedule (e.g. let us say 5 min behind as being late), transit passenger-occupancy is more than a defined threshold, the intersection is under saturated level, no queue spillback is happening, the signal did not have a priority request in the previous cycle, etc. It is more complicated than the unconditional priority because it needs more updated information about transit and intersections. Conditional priority will improve bus headway irregularity, crowding, and mean running time to almost the same levels as what absolute TSP. More importantly, conditional TSP makes transit running time less varied (less standard deviation of running time), which indeed improves the reliability of transit scheduling service. The performance of conditional TSP was studied and found that it is more effective for bus routes experiencing more severe lateness [47–49]. Meanwhile, person-based signal priority approach has been recently introduced optimizes signals and applies conditional transit priority based on transit and vehicle passenger-occupancy conditions [50].
