Active Transit Signal Priority

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.¹

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%,4 and delay was reduced up to 50% at target intersections.²

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 multimodal 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.

Evaluate and plan TSP based on the goals of minimizing person delay and improving transit reliability. When active TSP is implemented at an intersection, priority should be provided to as many passenger arrivals as possible, based on clearance intervals and crosswalk capacity for crossing pedestrians.

For transit corridors with short headways (under 6 minutes) it may be beneficial to operate conditional TSP, providing priority only to late vehicles. On corridors with longer headways or with high reliability, TSP should generally be applied to all transit runs, with time savings incorporated into the service schedule.³

Where transit vehicle volume is high enough that not all arrivals can be provided with priority, late runs, more full vehicles, or rapid services should be prioritized.

Far-side stops maximize TSP efficacy, since arrival at the signal can be anticipated more easily than dwell time. However, near-side stops may be preferable at transfer points.

In mixed-traffic operations, active TSP is often more effective at off-peak periods. It is most effective at moderately high traffic volume, with volume-to-capacity (v/c) ratios of 0.5 to 0.7, and remains effective up to a v/c ratio of about 0.9.⁴

Signal preemption should be applied with great care, as it requires transit vehicle arrival to be anticipated far enough in advance to provide a full pedestrian clearance interval.

Adaptive TSP incorporates continuous detection along transit corridors, not just at intersection approaches, and may be integrated with automatic passenger counters (APCs), communication among transit vehicles and signal controllers, and real-time traffic data to determine TSP interventions with better precision and reduce general traffic impacts.⁵

Common Transit Signal Priority Tools

Green Extension

provides extra time for a detected transit vehicle to clear an intersection. Green extension is most applicable when transit runs at the back of the vehicle queue, as is common at the first signal after a farside stop. Green extension may be the easiest form of TSP to implement on urban streets since it does not require unexpectedly truncating a pedestrian phase.

Green Reallocation

shifts when in the signal cycle the green phase occurs—if the transit vehicle is on pace to arrive late, the green phase begins and ends late to accommodate transit. Phase reallocation provides similar benefits to phase extension, but with less impact to cross street traffic since the total green time per cycle does not change. This strategy requires AVL technology.

Red Truncation

provides a green phase earlier than otherwise programmed, clearing an intersection approach with a waiting transit vehicle sooner than otherwise. Red truncation requires the detection of the transit vehicle far enough away that the crossing pedestrian phase can clear. It is easiest to implement on long blocks or on transitways with predictable travel times.

Upstream Green Truncation

also known as a reverse queue jump, stops traffic behind a bus as boarding is completed, allowing the bus to re-enter the lane after a pull-out stop. Upstream green truncation can also be used to stop traffic at an intersection where transit makes a far-side in-lane stop, preventing queuing in the intersection. Green truncation is most effective on moderate frequency transit routes where delay upon reentry due to congestion is common. It can also benefit passengers alighting and crossing the street behind the bus.

Phase Insertions and Phase Sequence Changes

describe the special bus-only phases or prioritization of turn phases used for shared turn/queue jump lanes, and are also helpful when buses make left turns.

Phase Reservicing

provides the same phase twice in a given signal cycle, such as a left-turn phase or a queue jump. Reservicing a phase can significantly reduce bus delay, particularly when the phase in question is relatively short.

Signal Treatment	Lane Types	Stop Types
Green Extension	Transit Lane, Transitway, Mixed Travel	Far-Side; Pull-Out or In-Lane
Green Reallocation	Transit Lane, Transittway, Mixed Travel	Far-Side; Pull-Out or In-Lane
Red Truncation	Transit Lane, Transitway, Shared Right Turn/Queue Jump	Near-Side or Far-Side; Pull-Out or In-Lane
Upstream Green Truncation	Mixed Travel	Near-Side or Far-Side; Pull-Out
Phase Insertion/Phase Sequence Chnage	Transit Lane, Transitway	Any
Phase Reservice	Transit Lane, Transitway, Mixed Travel	Any