Effects of variable cycle lengths on Transit Signal Priority operations

Abstract

Traffic signals in a coordinated network normally use a common cycle length which remains constant at all times, including when there is a request for priority from a public transport vehicle. This enables green waves to be maintained effectively but can limit the signals' ability to promptly serve the prioritised vehicle.

To study the effects of momentarily relaxing the constraint of cycle length during Transit Signal Priority (TSP) interventions, a new TSP system is developed for a CRSV halfstarre traffic signal controller, which permits a flexible cycle length during priority interventions. That system is tested using a Vissim microsimulation of a simple fictional network, and compared to the existing fixed-cycle-length TSP system included with the controller.

The new TSP system permits TSP actions as long as it is expected that the signal can return to its normal "in sync" timings within two cycles. During the intervention, the positive and negative impacts on each signal phase are monitored, and "Offset Correction Credits" are distributed, which each represent one second of additional green time. Signal phases which received extra time during the TSP intervention will receive negative OC Credits, and phases which were truncated will receive positive OC Credits. Once the intervention is complete, the signal will execute "offset correction" to return the signal to its normal "In Sync" timings while redeeming OC Credits.

The subject road network consists of fictional road with three coordinated traffic
signals, spaced 150 metres and 400 metres apart. The central intersection is the capacity-critical intersection and also includes a frequent bus line (12 buses per hour per direction) travelling along a median busway perpendicular to the coordinated direction.

In the scenario with a high flexibility to reduce green durations, the average delay for late buses dropped by 59% from 10.7 secondsto 4.4 seconds for the flexible-cycle system compared to the fixed-cycle system. With low flexibility, the average delay for late buses dropped by 78% from 28.0 seconds to 6.2 seconds. The large improvements in performance for buses are due to the flexible-cycle TSP systems being able to execute more TSP actions such as phase insertions which may not fit within a fixed cycle length.

However, the controller’s ability to remain in sync was negatively impacted and the frequency of queues exceeding storage increased by as much as 70% on short roadway links. However on long roadway links, delaysand queue lengths decreased in the coordinated directions thanks to the new TSP system’s green time compensation mechanism.
When the assumed occupancy rate for late buses is 50 passengers (corresponding to a busy but not overcrowded standard bus), there was no significant difference in person-delay between any of the scenarios. Early buses were not included in the calculation for average person-delay.