Transit signal priority

Transit signal priority (TSP) makes it more likely that the light is green when a transit vehicle reaches a signalized intersection. This strategy reduces travel times for transit vehicles by avoiding the need to stop and start at signalized intersections.

Key characteristics

Setting/Location

Corridor, Urban, Suburban

Technology

Collaboration

WSDOT regions

Northwest, South Central

Other names

  • Active Transit Signal Priority
  • Conditional Transit Signal Priority
  • Unconditional Transit Signal Priority
  • Passive Transit Signal Priority

Strategy description

Transit Signal Priority (TSP) adjusts the timing of a traffic signal’s red and green cycles to reduce the amount of time a transit vehicle spends waiting at a red light. 

TSP works by making minor modifications to the traffic signal cycle or phasing in the presence of approaching transit vehicles. It can either extend green times so an approaching transit vehicle can get through the intersection without stopping, or shorten red times so a stopped vehicle can get moving again.

TSP helps provide faster and more reliable transit service and is appropriate for any street-running transit vehicles, including buses, light rail, and streetcars.

The most common methods (or treatments) are:

  • Holding a green light (usually up to 10 seconds) for an approaching transit vehicle
  • Shortening a red light for a transit vehicle waiting at the light
  • Changing when a green phase will occur in the traffic signal cycle to time it to the arrival of a transit vehicle

Less common treatments include:

  • Giving a red light to upstream traffic so that a bus pulled off at a stop or station can re-enter the lane more easily
  • Providing special bus-only signal phases to help buses make left turns or “jump the queue” around traffic stopped at a red light

Unconditional & Conditional Priority

Setting rules for signal priority depends on the goals of the TSP system agreed to by both the transit agency and the signal operations agency. Unconditional priority rules mean priority is given to all arriving transit vehicles at all times. Agencies can also establish rules for conditional settings which allow more flexibility and consideration for schedules and traffic conditions.  

Common conditional rules include:

  • Schedule adherence (e.g. only when a bus is more than five minutes behind schedule)
  • Priority route/direction/time (e.g. only for inbound rapid routes during non-peak periods)
  • Occupancy (e.g. bus is at least 50% full)
  • Minimum time since previous TSP call (to give a coordinated signal enough time to return to coordination)
  • Congestion levels (only if traffic volumes are above or below a given threshold)

These conditional priority capabilities require various on-board and roadside devices to collect real-time information. For example, to determine whether a vehicle is behind schedule, the transit schedule system needs to be connected to an onboard GPS-based vehicle location device. Similarly, to estimate how many people are on board, the vehicle needs to be equipped with an automated passenger counter.

When to use this strategy

TSP makes sense for corridors where:

  • Transit routes are commonly delayed and travel time too unpredictable
  • Waiting at traffic signals is thea major source of delay for transit, as opposed to passenger loading/unloading delays or heavy traffic conditions
  • The corridor is often characterized by long distances between signals and/or long signal cycles
  • Block lengths are short and transit frequencies are high, but signal timings can be adjusted to match the typical travel speed of transit vehicles. This approach, referred to as “passive TSP”, is especially applicable in downtown environments.
  • Corridors that have bus rapid transit operating within them

Consider TSP’s impact on other traffic:

Because TSP systems interrupt a traffic signal’s standard signal cycle, agencies have to balance decisions about how frequently and under what conditions to use it. While it may help public transit run faster or closer to schedule, it may also cause delays for other traffic. A traffic signal often requires multiple cycles to return to its standard plan; if TSP is activated too frequently or in too short of succession, it can cause delays for other traffic. It may even prevent the signal cycle from being restored to its standard plan.

Strategy benefits:

  • Improves transit travel time and reliability by reducing the amount of time transit vehicles spend waiting at red lights

What you need in order to implement

Planning needs:

  • Cooperative involvement of transit and traffic signal agencies to develop a joint TSP program
  • Coordinated policies on the operating rules for TSP, what equipment is required, and how it will be implemented to ensure that the system is compatible across jurisdictions and used in common ways
  • Assessment of geometric and operational factors, including transit facility type, general traffic volume and capacity, signal spacing, and signal cycle length

Equipment needs:

  • Onboard technology to locate the vehicle, assess its current schedule status, determine whether to request signal priority, and communicate the request to the appropriate signal or signal system
  • Roadside or central technology to receive priority request communications and  determine whether and when to grant the request, as well as signal software to process the request and store the data

Learn more about this strategy

National Association of City Transportation Officials (NACTO) Transit Street Design Guide, Active Transit Signal Priority
https://nacto.org/publication/transit-street-design-guide/intersections/signals-operations/active-transit-signal-priority/

Federal Transit Administration (FTA) (2008), Transit Signal Priority Research Tools, FTA-CA-26-7049-2008.1 (PDF).
http://www.dot.ca.gov/newtech/researchreports/reports/2008/tsp_research_tools_final_report.pdf

About key characteristics

Location notes:

TSP works well for locations where there is high frequency transit services and people depend on transit to move.

Cost notes:

Costs to install and operate a TSP system are relatively low and vary based on the number of signalized intersections and transit vehicles outfitted with TSP equipment and the sophistication of the TSP solution implemented. Typically, the traffic signal operating agency pays for the equipment at the intersection and the transit agency pays for the equipment on-board the transit vehicles.

Technology notes:

Technology needs vary based on the design of the system but typically include onboard vehicle technology to locate the vehicle, determine its schedule status, and generate a priority request; wireless communications technology to relay requests between the vehicle and the intersection; and intersection technology to receive priority requests and modify traffic signal timing.

Collaboration notes:

TSP requires collaboration between the transit agency and the signal operator(s) along the transit route. These agencies must agree on overall system goals, as well as when and under what conditions priority for transit vehicles will be provided, what equipment will be installed and when the installation will occur, and who will be responsible for maintenance and repairs.