Providing priority for public transport vehicles at signalized intersections is an excellent way to reduce public transport travel time and increase schedule reliability, helping to make public transport more attractive and cost efficient. This article outlines public transport traffic signal priority systems.

Public Transport Priority at Traffic Signals

Public Transport Priority increases Attractiveness and Cost Efficiency

Travel time is one of the key variables that determines the attractiveness and cost of public transport service. Increasing the speed of buses and trams will:

• make service more attractive to customers - thus increasing ridership
• reduce the cost of providing service since fewer vehicles and operators are needed to provide a given level of service (alternatively, more service can be operated at the same cost).

So, speeding up public transport is a win-win situation: more ridership and less costs. There are many ways to increase public transport speed. In general, these techniques are called transit priority techniques or, when bundled together, Bus Rapid Transit .

Traffic Signal Priority

An important transit priority technique is providing priority to buses and trams at traffic signals. This means that when a bus or tram approaches a traffic signal, the traffic signal turns green, thus allowing the transit vehicle to proceed through the signal without stopping.

Traffic signal design (i.e. developing the 'phasing' patterns and exact amount of time provided to each 'phase' - referred to as 'timing') is relatively complex (a 'phase' is essentially the one specific pattern of traffic light indications, e.g. green light for northbound and southbound traffic).

Furthermore, the best traffic signal design should include careful design of the street geometry (lane pattern, dimensions, patterns for non-motorized traffic, etc.). Too often traffic signals and street layout are done separately, as a result neither works very well.

The following sections outline traffic signal priority approaches used in different cities and situations.

Dedicated Priority - Phasing Changes

The simplest traffic signal priority systems turn the signal green for public transport every time the vehicle approaches. This system adjusts the traffic signal 'phasing' when an approaching bus or tram sends a message to the traffic signal.  The action is not immediate, however, as the original phase must end before the signal turns green.  For example, if a pedestrian signal is underway,  enough time must be given for the pedestrian warning phase and to allow pedestrians to finish crossing the street.

These systems are not very popular because they can cause fairly significant traffic congestion in areas with sophisticated traffic signal control systems designed to move traffic efficiently (we can discuss whether this is bad or not later, but it's viewed as a problem for most traffic engineers and many others).

Longer Green Time - Timing Changes

In order to address the problem of traffic congestion caused by dedicated priority systems, another approach is to adjust the traffic signal 'timing' - the amount of time provided to the traffic signal 'phase' for the bus or tram, when the public transport vehicle sends a signal to the traffic signal controller saying that it would benefit from this added time.

For example, say that the bus is approaching a traffic signal that is green but about to turn yellow. The bus can send a signal asking the traffic signal to stay green a little longer so that it can pass through the intersection without stopping. Similarly, if the bus is approaching the intersection and the traffic signal has been red for a long time, it can send a signal asking the traffic signal to turn green a bit early, again eliminating the need for stopping.

In this example note how traffic engineering and traffic signal design must work together: if the bus needs to stop to pick-up or drop-off passengers on the corner ahead of the traffic signal (near side) then it makes little sense to extend the green light when a bus approaches since the bus may need to stop anyway. It is critical for traffic engineers and public transport planners to work closely in the physical design of stops/facilities and traffic signal systems.

Los Angeles has implemented this type of system on its rapid bus program. Their system is even more sophisticated than this description since it is linked to a computer that only gives the bus priority if the bus is running behind schedule. The goal is to keep the bus schedule reliable.

Phase and Timing Adjustment: Zurich, Switzerland

As part of the Zurich, Switzerland public transport priority program the traffic signals provide priority for buses and trams. The program has been in operation since the early 1980s and uses a highly innovative approach.

At first Zurich experimented with absolute priority systems (see above) but they realized that it was hard for drivers to send messages to traffic signals (it only involved pushing a button, but given all the other responsibilities drivers have, this type of system is unreliable). Furthermore, city traffic engineers also worried about the impact on vehicle traffic. So, Zurich developed its own system.

The Zurich approach combines an automatic prediction of when buses and trams will arrive at a traffic signal with a 'freestyle' traffic signal 'phasing' and 'timing' system. The automatic prediction system is straightforward: induction loops in the street sense when a bus or tram is passing over them and the travel time between the sensor and the intersection is known, thus the traffic signal recieves warning that the tram or bus is, for example, 30-seconds away.

There are several important aspects about this automatic system. First, notice that the system is hard-wired. Today one would probably use IT without wires, but the Zurich system was developed before IT technology was as inexpensive and easy to use as today. Second, because other transit priority measures have ensured that there is little delay on the route (e.g. maybe there is a dedicated transit lane), the time from the sensor to the intersection is known precisely. This is an important example of how the different public transport priority measures work together to create a whole that is greater than the sum of the parts.

Back to the traffic signal: the controler recieves the information that a bus or tram will arrive in, e.g. 30-seconds. Next, the traffic signal controller determines what the best combination of 'phases' and 'timing' would be so that the transit vehicle receives a green light exactly when it needs it. This means that green time at the intersection is not wasted; in other words, traffic, people or transit is always moving through the intersection. This is important because if nothing is moving through the intersection, people or bikes might just 'walk against the light' which would be a safety problem and interfere with the bus or tram.   Furthermore, automobile drivers would complain that transit priority was slowing down their trips (and you can be sure that drivers will find politicians who will fight transit priority on these grounds).

The interesting thing about the Zurich system is that it works with both 'phasing' and 'timing'. In other words the 'phasing' of the traffic signal changes to provide a green light for public transport exactly when the public transport needs it. Let's look at an example. Suppose that the normal 'phasing' of a traffic signal is: northbound (NB)/southbound (SB) left turns (LT), NB/SB through (TH) traffic, eastbound (EB)/westbound (WB) LT, EB/WB TH.  Normally these 'phases' repeat themselves in a cycle. Suppose that the phase that provides priority for public transport is NB/SB TH. Finally, suppose that the traffic signal is in the phase EB/WB LT when it receives information that the bus will arrive in 30-seconds. At this point, the traffic signal controller determines that it should change the pattern of 'phases' so that the bus receives priority (NB/SB TH) in 30-seconds. The traffic signal controller determines that there is enough time for the EB/WB TH 'phase' but that the NB/SB LT 'phase' must be skipped so that the bus does not need to stop. Then the controller implements the change and adds the NB/SB LT 'phase' after the NB/SB TH 'phase' before returning to the regular pattern.

The change in 'phasing' is relatively unique:  it means that the traffic signal pattern would not be consistent. This can be problematic when people are used to constantly repeating patterns.  Therefore, before Zurich implemented its system, the city switched traffic signal phasing to a random pattern and posted signs at all the intersections warning people about the change.

Summary

Many cities have implemented traffic signal priority systems. This article should be expanded and revised to include more examples and more technical information.

ALSO ON THE LIVABLE STREETS NETWORK

• Streetswiki:  Bus Rapid Transit
• Streetswiki:  Zurich, Switzerland
  

REFERENCES

Each source is referred to by the same number every time it is cited. Please keep citation style consistent.

[1] Nash, Andrew and Ronald Sylvia. Implementation of Zurich's Transit Priority Program. Mineta Transportation Institute, San Jose State University, Report 01-13, October 2001.

 

PICTURE REFERENCES

Pictures are cited in the order they appear above. Please keep citation style consistent.

[1] California Department of Transportation

[2]

 

FURTHER READING

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