Traffic Signal Priority


Priority can be given to public transport vehicles at traffic signals. This complements other priority measures in the running way such as reserved way, bus lanes, priority pull-outs, etc.

The primary objectives of priority for public transport at traffic signals is usually one or more of the following:

  • To reduce the time taken to pass through an intersection area. In urban areas, buses and trams may pass though many signalised intersections. The cumulative delay at all the intersections can add several minutes to the overall trip time, leading to passenger frustration and to increased resource requirements and operating costs. Buses are especially prone to delays at traffic signals. As they must stop for passenger boarding/alighting, and then may have to wait for a safe opportunity to rejoin the traffic stream, they often lose their place in the traffic ‘platoon’ from the previous intersection. Consequently, the traffic signal has turned to red by the time they arrive at the intersection. The objective is to better align the bus with the green phase, and thereby reduce delays.
  • To minimise the variance of time taken to pass through an intersection area. The amount of time taken to pass through the intersection depends on whether the signal is green or red, and if red, on when in the cycle the bus joined the queue. The cumulative effect over all the signalised intersections on a route can amount to several minutes. The impact can be even more severe if signal cycle times are lengthy. This variance can impact on the reliability and timekeeping and can lead to bunching. It may require extra time to be added to the schedule, thus adding to resource costs, Minimisation of variance is often the primary reason for implementing traffic signal priority for public transport vehicles.
  • To allow/assist buses and trams to safely make manoeuvres not permitted to general traffic. This is usually implemented in conjunction with other traffic management measures such as entrance to busways, bus-only streets, and contraflow bus lanes, or at cut-throughs at roundabouts.

The main techniques used are:

  • Adjust the stage or signal program phasing/timing: This is the most common technique. When the public transport vehicle is detected at or near the intersection:
    • If the signal is green, the green phase can be extended for several seconds until the vehicle has passed through
    • If the signal is red, the phases can be shortened so that the public transport vehicle gets a green light at the earliest opportunity
    • In some cases, public transport vehicles may be given ‘zero wait’ priority – this is usually only done for trams, but may also done for BRT. In this case, the phases are extended or truncated sufficiently quickly that the light is always green when the vehicle arrives. The vehicle detection point needs to be located at an appropriate distance to allow sufficient time for the phase changes to be done safely.
  • Adjust the stage phasing: This technique changes the sequence of the phases. It may omit a particular phase (e.g. turning filter for other traffic, or pedestrian phase), or bring forward a phase that gives the needed priority to the public transport vehicle. Traffic engineers are often reluctant to use this technique. The reason is that motorists become familiar with the phasing of various intersections. They may move in anticipation of an expected green light or filter, which can lead to accidents if the sequence changes and they get it wrong.
  • Bus-only turns: Public transport vehicles may be permitted to make turns or go straight ahead where it is prohibited for other vehicles. In some cases, this may apply only at certain times of the day. The traffic signal aspects and approach signage need to indicate clearly that such movements are permitted only to the public transport vehicles.
  • Bus-Gating: This is a technique used to allow the public transport vehicle to move through the intersection ahead of the other traffic. It is usually implemented with a physical priority measure that enables the vehicle to get to the stop-line level with, or ahead of, the other traffic. For light rail operating on dedicated rights of way, this can simply be it’s own right of way. For trams or buses, a short dedicated lane may be provided. To overcome geometry problems at junctions, a second signal may be installed some distance back from the primary signal, so that the bus gating can occur there and the full junction can be utilised by all traffic. The bus gating has a special signal for buses, which gives the green a few seconds ahead of the other traffic. This is sufficient for the bus or tram to be at the front of the platoon and first through the intersection.
  • Bus-only phases: Some phases may be called up only when a public transport vehicle approaches. This would be typically be used where public transport vehicles need to enter general traffic from a restricted running-way, or to allow a manoeuvre permitted only to the public transport vehicles (e.g. entering a busway or contraflow bus lane).

Priority at traffic signals may be selective – i.e. not all public transport vehicles receive the priority facility. The main cases are:

  • Route specific requirements, granted only to routes making a specific manoeuvre.
    • Filter to turn across oncoming traffic
    • Entering busway
  • Mode specific priority, where the level of priority for one mode is higher than for others. This is particularly used for trams and BRT trunk routes.
  • Schedule position. Public transport vehicles may be granted priority when they are on-time or in delay, but not granted priority if they are running ahead of their scheduled time.
  • Conflict priority, where two or more public transport vehicles may seek priority at the same intersection. The assignment of priority includes:
    • Prioritise by mode, where trams and BRT vehicles get priority ahead of others
    • Prioritise by route, where one route is given priority over others
    • Prioritise by manoeuvre request, where one type of request such as filter turn across oncoming traffic is given priority over simpler requests such as early green
    • Prioritise vehicle delay status
    • First come, first served

All traffic signal priority also involves a balancing of interests between the public transport vehicles, general traffic and pedestrians.

Technologies, data and resources

Traffic signal priority utilises the available traffic signal control systems. Most modern systems already have inbuilt capability for providing priority. The priority phases are generally set up as for any other phase. The additional software capacity deals with handling the priority requests, application of the priority rules and managing the staging/timing.

The additional technologies used for traffic signal priority relate to the vehicle detection, so that the traffic signal controller is aware of the request and its associated information. The main means of detection are:

  • Loop in the roadway or radar detects vehicle presence and can identify from the profile that it is a bus or tram. The controller deduces from loop location what priority to give.
  • Transponder in the roadway or at roadside dialogues with counterpart on the public transport vehicle. Information from the vehicle assists the signal controller to deduce what priority is requested.
  • Information from the CAD/AVM or other vehicle location system advises the traffic control system of approaching vehicles and the priority requests
  • Optionally, a detector may be placed within or beyond the junction to verify that the public transport vehicle has passed through (“departure loop”). This can allow the phase to be terminated at the earliest opportunity.

Advantages and Cautions

The primary advantages of Traffic Signal Priority for public transport vehicles are to:

  • Reduce delays at intersections, and hence reduce overall running time
  • Reduce variance of time spent stopped at traffic signals, and hence improve service reliability
  • Reduce vehicle resource requirements and operating costs
  • Improve customer satisfaction

The principal cautions in relation to Traffic Signal Priority for public transport vehicles are:

  • Priority cannot be given to everyone at every time. Reasonable balance is required between the public transport vehicles and general traffic
  • When frequencies are very high and collective headways are lower than cycle lengths, only a limited percentage of bus or tram trips will be eligible for receiving priority.
  • Good working relationships are required with the traffic authorities
  • Accurate location of the vehicles and proper location of loops/transponders can be a time-consuming activity, while maintenance of the hardware, especially loop detectors can be costly. Failure to do this correctly can result in not getting the desired priority and/or imposing unnecessary delays on other traffic

Relevant Case Studies

Dublin, Zurich