Traffic Light Priority: ITS Evaluation Guide

Published August 14, 2022 in ITS Evaluation Guide

Traffic light management systems were extensively rolled out in the 1970s and 1980s. For many years, traffic light control was at the heart of ITS. These systems were developed to reduce congestion by improving the traffic flow and driver experience through cities.

However, for emergency services, traffic lights increased their travel times. Traffic light priority was developed to give emergency services, including police, ambulances, and fire services, priority through an intersection safely. The traffic light priority concept was further developed and applied to public transport as authorities wanted passengers to shift modes from cars to buses or trams.

Traffic light priority works when a vehicle sends a signal to a traffic light controller at an intersection or a central control system. The controller or the central control system then adjusts the traffic light phases to prioritise the vehicle over other traffic through the intersection. Typically, a green phase may be extended to let the vehicle through. If it faces a red signal, this phase may be shortened, so a green signal occurs earlier.

Traffic light priority can always be on, allowing the vehicle to always have preference through an intersection. This is great for a Bus Rapid Transit (BRT) system, which may need specific priority to cross an interchange, but causes needless disruption for traffic if a bus is running on time or if an ambulance is not on an urgent job.

The more refined way is to request priority if the bus or tram is late. Here, the schedule adherence system sends the request signal to the traffic light controller or host, indicating if the vehicle is on time, late, or ahead of schedule. Depending on the vehicle’s on-time status, the request may then be granted. For example, priority may only be given to late vehicles. Traffic light priority generally works best for dedicated bus lanes like BRT systems or light rail in mixed traffic.

Benefits

Traffic light priority benefits include:

Reliability and punctuality. Traffic light priority can significantly improve on-time performance and improve service reliability.

Mode displacement. For cities trying to promote the switch from private cars to public transport, traffic light priority can be used to prioritise public transport modes like buses and trams, creating faster travel times when compared to cars.

Improved resource utilisation. The two major cost areas for a public transport operation are vehicles and drivers. If traffic light priority is implemented, you can decrease travel times. When optimising schedules with shorter travel times, authorities can also reduce the number of vehicles required.

Better adaption to disruptions. If you use traffic light priority to prioritise late vehicles (see A Re Yeng Case Study), the system can be used to make up the time lost to disruptions without making early services earlier – providing a more reliable service.

Evaluation Guide

When assessing a traffic light priority system:

1. Does your jurisdiction have a policy to encourage public transport using vehicle priority?
If your city or transport authority has a policy promoting public transport to shift commuters from cars to buses, or other modes, then traffic light priority could be a great policy tool to improve travel times. If you don’t have a policy to promote public transport, consider creating one first.

2. Is the system automatically triggered, and are the activation thresholds easily configured?
Drivers need to focus on driving, so it is important that a traffic light priority system is triggered automatically without driver intervention. Priority requests should be triggered by a combination of factors, such as vehicle location and schedule adherence. For example, if a bus is 200 metres from an intersection and three minutes behind schedule, the AVLC system might automatically trigger a priority request. The activation thresholds like time and location should have an easy-to-use configuration interface. There should also be reports available to analyse the success of the various changes that allow you to make informed decisions based on the results.

3. Do you want a centrally controlled system, or a decentralised system?
This depends on the capability of your existing traffic control system. Centrally controlled systems have communications to each controller from the control centre and try to optimise traffic flow across multiple intersections. Public Transport priority requests are centrally brokered and sent to the intersections as part of a holistic traffic plan. This gives priority without major disruption to the overall traffic flow.

Decentralised systems mean that the bus communicates direct to the intersection without consideration of the bigger picture. This can work well but the connection to the intersection can experience radio interference and sometimes requires moving antennas to receive the best reception. This does not try to synchronise with the broader traffic flow, so a priority request may have a larger knock-on effect on traffic. Both systems work well once set up, but each has implementation challenges and ultimately you need to work with what is already in place.

4. Do you wish to use traffic light priority strategically to encourage a public transport mode shift?
Public transport system success depends on encouraging people to move from one mode of transport to another, i.e., switching from a private car to a bus or tram. If you can provide a faster and more convenient option by giving public transport priority over other traffic, then public transport is more attractive, and many commuters will shift transport modes.


Traffic Light Priority Case Study: A Re Yeng BRT Traffic Signal Priority, Tshwane, South Africa

The City of Tshwane Bus Rapid Transit System makes use of traffic signal priority to keep its bus system operating on time. The system installed in Tshwane is a decentralised system, where each bus can directly signal an intersection controller on the route. The schedule adherence system initiates the traffic signal priority onboard the vehicle, and using the rooftop mounted antenna, then signals the intersection controller.

Tshwane has focused on service reliability, rather than speed. Traffic controllers only activate the bus priority through an intersection when it is over five minutes late.

The longer seven-kilometre trunk routes from the north of the city experience high congestion levels during the morning peaks. There are several sections where limited space under bridges cause buses to transition from dedicated lanes into mixed traffic and back. This section can be prone to delays and disruptions.

By making use of the traffic signal priority system, buses can recover up to 15 minutes from delays, and services run on average 17 minutes faster during peak traffic. The result is an extremely on time and reliable service, even when there are unexpected delays on the route. This promotes modal shifts, with the result that commuters demanded that the city implemented park and ride service much earlier than initially planned.

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Andrew Shaw

ITS Project Manager, Trapeze Group

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