**3. Signal control systems and TSP**

To better understand how to treat transit differently, diverse types of signal control systems need to be explained. Traffic signal system is one of the most significant parts of transit signal priority application in the emerging system of smart cities. Signals have been designed to control demand so as to improve traffic flow. There have been many developments in signal control systems. In this section, signal control models are categorized into four generations including: fixed time, coordinated-actuated, fully actuated, and adaptive signal control.

*Pre-timed (fixed) signal control* means that each phase has a fixed split length, resulting in a signal with a fixed cycle length. To be more responsive to traffic changes, one approach could be to use different plans according to the time of day (a.m. and p.m. peak, midday, nighttime, etc.). This way, the historical traffic demands will be used to determine signal timing plans.

The TSP on the fixed time control logic indicates whether the transit is projected to pass through the signal at a green light. If so, no alternative is made; but if the bus is projected to be at the stop line just after the end of green, green extension tactic will extend the green time until the transit vehicle can pass through or before the allowable maximum green. The green time required for an extension is taken from the next phase or other conflicting phases (if there are more than two critical phases). However, if the bus is at the signal while traffic from another approach is being served, the TSP logic truncates the active green phase, after the minimum green of that phase is satisfied. In the fixed-time control logic, applying TSP will reduce bus delay substantially, whereas it may increase the delay of the conflicting phases. Lack of compensation in the fixed time signal control does not allow it to recover from interruptions like TSP. That is part of the reason why many developed models have limited applying TSP over the fixed-time control at every cycle [17].

*Coordinated-actuated signal control* is another controlling system that is mostly being used for signals placed along a corridor. The logic provides all signals with a fixed cycle length and let non-coordinated phases behave like actuated control, aiming to enlarge green bandwidths and allow all slacks to run in the coordinated phases. Cycle length, force-off points, offsets, and phase sequences are mostly the signal timing parameters that are being optimized widely through available signal optimization software like Synchro and Transit-7F Error! Reference source not found.. There have been some optimization models developed over coordinated-actuated control to make its performance even better which can be found in [19, 20].

Applying TSP to coordinated-actuated logic is done by granting green extension to the coordinated/transit phase, early green to the actuated phases (non-coordinated phases), and TSP phase rotation whenever it is needed. As the cycle length is fixed, like the fixed-time control, the granted green extension time is taken from other conflicting phases. Actuated control with absolute priority can result in near zero delay for busses, but it sometimes causes long delays for the general traffic [21].

*Actuated signal control* relies on traffic data from sensors embedded in the infrastructure including loop-detectors, video detectors, or radar, to make controlling decisions. Actuated control better captures the real-time dynamic of traffic system since traffic demand may fluctuate from time to time. The fully actuated signal control run as fast (snappy) as possible to have less slack time, cycle length, and thereby less overall delay at intersections [22]. It matches supply to demand in real time. It has a feature of compensation which means if the controller gives more green time to a phase due to considerations such as TSP; the logic automatically will compensate and provide more green time to the conflicting phases in the next cycle. The faster it runs, the more efficient it will be [23–25]. The actuated traffic signal

**187**

*Transit Signal Priority in Smart Cities*

(oversaturated condition).

well with signal priority [18].

**4. Passive versus active**

used for emergency vehicles [41].

*DOI: http://dx.doi.org/10.5772/intechopen.94742*

functions approximately as a fixed signal when the degree of saturation is too high

*Adaptive signal control* gets feedback from detectors based on the latest update of the past 5 or 10 minutes to update and re-optimize the control plans. Adaptive control can also be designed to predict traffic flow and optimize in the anticipation of the flows expected to arrive in the next few minutes (e.g. 2 to 5 min horizon). There are several adaptive control systems being developed; some of which include: SCATS [26], OPAC [27], TRANSYT-7F [28], UTOPIA [29], SCOOT [30], RHODES [31], ACS-Lite [32], MOTION [33], and more programs are also coming on to the market. Many of them are adaptive signal systems with a centralized controller. In adaptive signals with centralized control, the complexity increases as the number of traffic lights and contributing variables increase. Adaptive control with decentralized approach has also been developed, e.g. self-organizing system, that functioned

The use of TSP on top of adaptive signal control was developed by many scholars. TSP applied to SCATS includes green extension, special phase sequences, and compensation to the non-transit phases [34]. Transit priority on TRANSYT-7F benefited bus delay-saving by 6 s/intersection/bus [35]. TSP on UTOPIOA reported a 20 percent increase in the average bus speeds [36]. TSP with SCOOT reached bus delay-savings ranging from 5 to 10 s/signal [35]. Priority on RHODES has also increased traffic speed and reduced average and variance of bus-delay significantly [37, 38]. TSP with self-organizing system result in a very low bus delay [18, 39].

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

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

#### *Transit Signal Priority in Smart Cities DOI: http://dx.doi.org/10.5772/intechopen.94742*

*Models and Technologies for Smart, Sustainable and Safe Transportation Systems*

coordinated-actuated, fully actuated, and adaptive signal control.

demands will be used to determine signal timing plans.

To better understand how to treat transit differently, diverse types of signal control systems need to be explained. Traffic signal system is one of the most significant parts of transit signal priority application in the emerging system of smart cities. Signals have been designed to control demand so as to improve traffic flow. There have been many developments in signal control systems. In this section, signal control models are categorized into four generations including: fixed time,

*Pre-timed (fixed) signal control* means that each phase has a fixed split length, resulting in a signal with a fixed cycle length. To be more responsive to traffic changes, one approach could be to use different plans according to the time of day (a.m. and p.m. peak, midday, nighttime, etc.). This way, the historical traffic

The TSP on the fixed time control logic indicates whether the transit is projected to pass through the signal at a green light. If so, no alternative is made; but if the bus is projected to be at the stop line just after the end of green, green extension tactic will extend the green time until the transit vehicle can pass through or before the allowable maximum green. The green time required for an extension is taken from the next phase or other conflicting phases (if there are more than two critical phases). However, if the bus is at the signal while traffic from another approach is being served, the TSP logic truncates the active green phase, after the minimum green of that phase is satisfied. In the fixed-time control logic, applying TSP will reduce bus delay substantially, whereas it may increase the delay of the conflicting phases. Lack of compensation in the fixed time signal control does not allow it to recover from interruptions like TSP. That is part of the reason why many developed models have limited applying TSP over the fixed-time control at every cycle [17]. *Coordinated-actuated signal control* is another controlling system that is mostly being used for signals placed along a corridor. The logic provides all signals with a fixed cycle length and let non-coordinated phases behave like actuated control, aiming to enlarge green bandwidths and allow all slacks to run in the coordinated phases. Cycle length, force-off points, offsets, and phase sequences are mostly the signal timing parameters that are being optimized widely through available signal optimization software like Synchro and Transit-7F Error! Reference source not found.. There have been some optimization models developed over coordinated-actuated control to make its performance even better which can be

Applying TSP to coordinated-actuated logic is done by granting green extension to the coordinated/transit phase, early green to the actuated phases (non-coordinated phases), and TSP phase rotation whenever it is needed. As the cycle length is fixed, like the fixed-time control, the granted green extension time is taken from other conflicting phases. Actuated control with absolute priority can result in near zero delay for busses, but it sometimes causes long delays for the general traffic [21]. *Actuated signal control* relies on traffic data from sensors embedded in the infrastructure including loop-detectors, video detectors, or radar, to make controlling decisions. Actuated control better captures the real-time dynamic of traffic system since traffic demand may fluctuate from time to time. The fully actuated signal control run as fast (snappy) as possible to have less slack time, cycle length, and thereby less overall delay at intersections [22]. It matches supply to demand in real time. It has a feature of compensation which means if the controller gives more green time to a phase due to considerations such as TSP; the logic automatically will compensate and provide more green time to the conflicting phases in the next cycle. The faster it runs, the more efficient it will be [23–25]. The actuated traffic signal

**3. Signal control systems and TSP**

**186**

found in [19, 20].

functions approximately as a fixed signal when the degree of saturation is too high (oversaturated condition).

*Adaptive signal control* gets feedback from detectors based on the latest update of the past 5 or 10 minutes to update and re-optimize the control plans. Adaptive control can also be designed to predict traffic flow and optimize in the anticipation of the flows expected to arrive in the next few minutes (e.g. 2 to 5 min horizon). There are several adaptive control systems being developed; some of which include: SCATS [26], OPAC [27], TRANSYT-7F [28], UTOPIA [29], SCOOT [30], RHODES [31], ACS-Lite [32], MOTION [33], and more programs are also coming on to the market. Many of them are adaptive signal systems with a centralized controller. In adaptive signals with centralized control, the complexity increases as the number of traffic lights and contributing variables increase. Adaptive control with decentralized approach has also been developed, e.g. self-organizing system, that functioned well with signal priority [18].

The use of TSP on top of adaptive signal control was developed by many scholars. TSP applied to SCATS includes green extension, special phase sequences, and compensation to the non-transit phases [34]. Transit priority on TRANSYT-7F benefited bus delay-saving by 6 s/intersection/bus [35]. TSP on UTOPIOA reported a 20 percent increase in the average bus speeds [36]. TSP with SCOOT reached bus delay-savings ranging from 5 to 10 s/signal [35]. Priority on RHODES has also increased traffic speed and reduced average and variance of bus-delay significantly [37, 38]. TSP with self-organizing system result in a very low bus delay [18, 39].
