APPLICATION & CONTEXT
Where signals are a major source of delay for transit, particularly when signal delay is a significant portion of transit delay even at times or locations when traffic congestion is not a primary issue. For TSP to work, transit vehicles must be able to reach a signal, either with a dedicated lane or transitway, or by making use of an otherwise clear lane.
Corridors with relatively long signal cycles, or relatively long distances between signals, are good candidates for active TSP.
Specific intersections with long signal cycles or that favor the cross street and operate off of the progression of the rest of the corridor provide strong benefits.
Where transit routes turn, active TSP can extend turn phase time or reservice a turn phase to provide a clear turn lane and additional phase time for slow maneuvers.
Conditional TSP strategies are easiest to implement with moderate to long headways, allowing the signal cycle to gradually return to its non-priority timing. On higher-frequency corridors, decide whether to provide signal priority at every signal cycle, or only to late vehicles.
In most applications, when a transit vehicle receives a priority phase (~10 sec), the dedicated time is restored by deducting time from up to four subsequent cycles. If headways are too short, the signal cycle can never be restored and may cause adverse delay.
Harriet R Smith, P Brendon Hemily, and Miomir Ivanovic. Transit signal priority (TSP): A Planning and Implementation Handbook. ITS America, Washington, DC: 2005.
For BRT and LRT, which often use larger vehicles to increase headways and reduce costs on high-capacity routes.
Unconditional signal priority or preemption should be considered at actuated signals.
Active TSP can reduce transit delay significantly. In some cases, bus travel times have been reduced around 10%, and delay was reduced up to 50% at target intersections.
TSP applications using AVL technology was demonstrated to reduce total bus trip times during peak hours between 4 and 15% in Minneapolis. Applications in Portland, Seattle, and Los Angeles noted 8–10% travel time decreases.
Jia Hu, Byungkyu (Brian) Park, and A. Emily Parkany. Transit Signal Priority with Connected Vehicle Technology. Transportation Research Record 2418, Journal of the Transportation Research Board, Washington, DC: 2014.
A number of studies of TSP implementation on streetcar routes in Toronto recorded widely varying travel time improvements, even up to 50% reductions in delays at some intersections. Factors such as stop siting, service frequency and ridership, and separation from traffic all impacted TSP effectiveness in reducing spot delay.
Danaher, Alan R. Bus and Rail Transit Preferential Treatments in Mixed Traffic. TCRP Synthesis 83, Transportation Research Board, Washington, DC: 2010.
TSP is effective at intersections with routinely long queues, or on commonly delayed transit routes. TSP is most effective at intersections with a far-side stop or no stop, allowing the bus to clear the intersection without waiting at a signal
Signal priority usefulness depends on both geometric and operational factors like transit facility type, general traffic volume and capacity, signal spacing, and cycle length.
Active TSP may increase waiting times on cross streets, an especially important factor when transit lines intersect.
At high transit volumes, consider pre-timed strategies such as transit signal progressions. On streets with short distances between signals, a low-speed fixed signal timing strategy may confer more benefits to transit and multi-modal traffic than active TSP.
Active TSP requires a high degree of coordination between the agencies responsible for signals and transit vehicles and operations, with regard to on-board technology as well as signal technology and communications systems, transit schedules, and system goals. Coordination needs may require long-term agreements and planning of vehicle and signal equipment purchases based on goals, since not all equipment can perform all functions.