Traffic Signals

Welcome to the ACT’s Traffic Signal info webpage. The following webpage is designed to answer some common questions asked about traffic signals.

Canberra’s first two traffic signals were brought into operation on the same day, 23 October 1965. They were located at the intersections of Northbourne Avenue & London Circuit and Northbourne Avenue & Cooyong Street.

The ACT now has around 275 signalised intersections under the central control of SCATS (Sydney Coordinated Adaptive Traffic System). SCATS is used around the world and was developed by the Road Traffic Authority of NSW. Some of the functions performed by SCATS include:

• Coordination of traffic signals;
• Fault reporting (for example blown traffic signal lamps); and
• Data recording (traffic volumes at intersections).

The following sections will help answer some of the questions you may have regarding traffic signals.

Why have Traffic Signals

The main reasons for traffic signals are:

• To allow vehicles and pedestrians to negotiate an intersection safely and efficiently;
• To be able to respond to varying traffic patterns throughout the day; and
• Through coordination, large volumes of traffic to travel along a road corridor with minimum delay.
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How Traffic Signals Work

A signal controlled intersection comprises of:

• A traffic signal controller;
• Signal lanterns that display the different colours to both vehicles and pedestrians;
• Vehicle detector loops and pedestrian push buttons that allow information on vehicle and pedestrian movements to be registered; and
• The underground electrical cables that connect all the components together.

The following sections describe some of these components.

The Controller

The controller (housed in a grey metal box on a corner of the intersection) is the 'brains' of the system.  It is effectively a computer that is the fundemental means of controlling all vehicle and pedestrian signal displays and ensuring green signals cannot be displayed simultaneously to conflicting traffic movements.  It processes information received from the detector loops and pedestrian push buttons and allocates right of way to the competing vehicle and pedestrian movements in accordance with its programming.  It determines the length of the green signal for each traffic movement and controls the transition from one combination of green and red displays (known as a phase) to the next.  It can operate in a 'stand alone' manner or under instruction from a central computer system that allows the coordination of a series of intersections.

Traffic Signal Lanterns

Vehicle lanterns come in two sizes - 200mm and 300mm diameter.  200mm lanterns are generally used for 60km/hr roads and under and 300mm lanterns are generally used for 70km/hr roads and above.  Modern pedestrian lanterns use the symbolic walking green and standing red figures although there are still older lanterns around that display 'WALK' and 'DONT WALK'.  Modern signal lantern optics use LED's (Light Emitting Diodes) which are very energy efficient and long lasting.

Vehicle Loop Detectors and Pedestrian Push Buttons

The loops and pushbuttons are the means by which the controller assess the varying demands for green time at the intersection. Vehicles are detected using loops of wire that are buried in the road just before the stop line at the traffic signals. When a vehicle passes over the loop the inductance (magnetic field) of the loop changes and the vehicle is recognised by the controller.

For pedestrians, the controller knows there are pedestrians wanting cross when a push button is pressed. When the 'WALK' signal starts some push buttons produce a series of fast beeps. The fast beeps let visually impared people know its time to cross the road. When the 'DONT WALK' signal is displayed the same push buttons produce a series of slow beeps. Visually impared people use these beeps to locate the push button.

Signal Phases and Cycles

Each combination of greens and reds that the controller is allowed to display is called a phase.  Each phase has a programmed minimum time such that once the signals have entered a phase they cannot change again until the minimum timers have expired.  The sequence of phases that ultimately allows all the vehicle and pedestrian movements at the intersection to proceed is called the signal cycle.

The transition sequence from each phase to the next is predetermined.  First the signals for traffic losing right of way change from green to yellow and the yellow signal is displayed for a fixed period.  The yellow signals then change to red.  There is then a delay before the signals for traffic gaining right of way change to green.

The length of the yellow signal depends on the speed of the road.  The yellow means stop unless unsafe to do so - yes it does mean this and no, it does not mean go faster. The yellow signal is timed such that no one travelling at the speed limit approaching a green signal that changes to yellow should be in a position such that they are unable to stop before the signal changes to red.

The delay after the red is displayed before the start of the opposing green is based upon the size of the intersection and speed of the road. The idea behind the length of this delay, sometimes called the all-red time, is to cover the worst case of a vehicle entering the intersection late during the yellow.  If this were to happen the delay allows that vehicle to be clear of the path of a vehicle just about to enter the intersection on the next green. As you can see the larger the intersection the longer this all-red time has to be.

The yellow and all-red times for each intersection are determined in accordance with national guidelines. As the times are based upon the physical size of the intersection and the speed of the road the times do not vary.
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Modes of Operation

Traffic signals can operate in two main modes of operation – coordinated or isolated.

In isolated mode the traffic signals respond to traffic demands from the traffic signal loop detector located just before the stop line at the traffic signals.

When a vehicle is above the loop detector the traffic signals “see” the vehicle and places a “call” in for the direction the vehicle is travelling. When a vehicle pulls up on a side road the traffic signals “see” the vehicle and if there are no cars on the main road detectors the signals change phase to the side street phase. Once no more vehicles are seen by the detector(s) on the side street the signals change back to the main street phase.

This mode of operation works very well for traffic signals that either have low volumes of traffic, no major flow of traffic through the intersection or for traffic signals that are far apart.

The other mode of operation for traffic signals is coordinated. In this mode the traffic signals provide progression for traffic as they travel along a particular route.

For traffic signals to be coordinated they need two things:
1. A common cycle time. The cycle time is the time it takes for the traffic signals to go from the start of the main phase to the other phases and back to the main phase; and
2. An offset between the start of one intersection’s main phase and the next intersection’s main phase.

The benefit of coordinating signals is that a large volume of vehicles can pass through multiple signals with minimum delay. The disadvantage is that the common cycle time for the coordinated signals is set by the largest and most complex intersection. This usually means that the smaller intersections near larger intersections could change phases faster but are forced to "run" slower. This is the main reason traffic signals can seem to take so long to change phase. The traffic signals stay green for the main road so that the potential platoon (group) of vehicles travelling down the road get a green at each set of lights.

When coordinating traffic signals a balance must be maintained between allowing the progression of a platoon of vehicles and keeping the wait times for side street vehicles to a minimum.
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Filter Turns

Some drivers in the A.C.T dislike the use of red arrows to prevent drivers turning right, even when there are considerable gaps in the flow of oncoming traffic. The ACT uses the same national standards to determine the use of arrow signals as other states in Australia.

The reason we have so many red arrows is because of safety, and basically stems from the high quality road system in the ACT. Many of our arterial roads are multi-lane with a speed limit of 80kph. Research clearly shows that the higher the speed of the oncoming traffic and the wider the road that has to be crossed, the more difficult it is for a right turning driver to choose a safe gap in oncoming traffic. To try and reduce the frustration that this can cause for some drivers when traffic volumes are light, we are endeavouring to make the lights change more quickly under these circumstances.
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Pedestrian Crossings at Traffic Signals

A common misconception is that people have to get all the way across the road while the Walk signal is on. This is not true. The purpose of the Walk signal is to inform pedestrians when they can start to cross the road.

It is the flashing Don't Walk and steady Don't Walk signals that are timed to allow enough time for pedestrians to cross the road safely.

The way the pedestrian signals are set up is if a pedestrian leaves the sidewalk at the very end of the Walk signal they will still have enough time to safely cross the carriageway they are on.
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Roundabouts vs Traffic Signals

Traffic signals are relatively expensive to install / operate. They are normally only considered for intersections with particularly high levels of traffic. Roundabouts, by comparison are cost effective to install / maintain and work well in low levels of traffic. Traffic engineers use the Australian Standard AS1742 and Austroads Design Guidelines manuals to assess whether traffic flow will be improved at particular intersections with the installation of signals or whether the construction of a roundabout would be more useful.

Some people argue that roundabouts are preferable to traffic signals. It is important to note that traffic signals do have some advantages over roundabouts. Traffic lights generally require less land than roundabouts and can have specific facilities for pedestrians. Additionally, roundabouts have an in-built priority rule, which means that in heavy traffic, movements in one direction can tend to dominate and cause excessive delays to vehicle movements in other directions. Alternatively, traffic signals can be programmed to give priority to one direction over another direction. A report published in the UK also suggests that roundabouts are 2-3 times more dangerous for cyclists than traffic lights.

In light to medium traffic conditions, roundabouts can cause less delay to traffic movement than traffic lights. In these instances the relatively low maintenance costs of roundabouts will mean that their installation is preferred.

The installation of signals or the construction of a roundabout at a particular intersection is ultimately based on a thorough assessment of traffic flow, available land, intersection accident history and the intersections overall place in the transport network.
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Reporting Problems / Concerns

If you do see a problem with the traffic signals or you want to ask a question about the traffic signals, the traffic signal team would be more than happy to help.

To report faults / concerns you can either go online and submit a question or feedback via the Canberra Connect website or phone Canberra Connect on 13 22 81. Canberra Connect will then pass on the fault / concern to us at the Traffic Signals Section.

When reporting a fault or raising a concern please try to include as much information as possible. For example we sometimes get reports of “signals not working”. We don't know if this means all the signals were not working as in a power failure, or just one lantern was blown. The fault may even be that the lights don't change for people on the side street. When reporting a fault / concern it helps if you can provide:

• The location of the traffic signals - At the intersection of Northbourne Ave & Barry Drive.
• The details of the fault / concern – Heading towards Civic along Barry Drive the left lantern was blown.
• When did it happen / what time did you notice the fault – Around 5pm on Wednesday.
• Any other information that may be relevant.
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Frequently Asked Questions

How come Drakeford Drive isn’t coordinated!!!

The simple answer – it is – for the major flow of traffic along Drakeford Drive. The following section will show that in order to give good coordination to one direction of traffic along Drakeford Drive the other direction has to be disadvantaged.

Below is an illustration of a time / distance diagram that displays the AM coordination of Drakeford Drive.

Think of the diagram as a graph. The x-axis represents distance along Drakeford Drive.  The numbers along the bottom, 100, 107, 111, 56, etc, are the reference numbers of the different signalised intersections, with 100 being Erindale Drive, 111 being Athllon Drive, 64 Sulwood Drive, etc.  They are plotted to scale so the distance between the intersections on the graph is proportional to the distance between the intersections on the road.

The y-axis represents time.  Plotted vertically at the location of each signalised intersection is the sequence of greens and reds seen by drivers on Drakeford Drive over several cycles of the lights.

It is possible to draw a diagonal line starting at the beginning of a green at Erindale Drive with a slope that represents the speed (80km/hr) of traffic along the road.  As you move up that diagonal from left to right, ie travel along Drakeford Drive from Erindale Drive to Sulwood Drive, you can see the colour of the lights as you reach each signalised intersection.  The diagram represents the morning peak coordination and as such traffic heading north receives good coordination. As the traffic approaches each intersection the traffic signals are green allowing the vehicles to proceed through the intersection without stopping.

However, imagine you are travelling southbound during the morning peak.  This time start your diagonal line at the beginning of a green at the Sulwood Drive lights and draw it with the same slope but from right to left.  You can see that as you arrive at the first set of lights they are just about to turn red. If you are in the first vehicle in the platoon you might just get through but otherwise you will hit a red and have to stop. Move up the vertical red and then set off again on the diagonal at the start of the next green.  Red again at the next set of lights, and so on.  This is the sequence of lights if you are travelling in the opposite direction to the major traffic flow.

What the diagram illustrates is that it is relatively easy to provide coordination in one direction, but what happens in the other direction depends on the speed of traffic and the spacing of the intersections.  The only way to improve the coordination for the southbound traffic is to spoil the coordination of the northbound traffic. With a ratio of 2:1 vehicles heading north verses south in the morning, the northbound traffic receives the better coordination.
 
In the afternoon peak the situation is reversed. Good coordination is provided for southbound traffic but similar problems occur for drivers travelling north.

As it can be seen from above it is the spacing of the signalised intersections (distance between the signalised intersections) and the speed limit of the road (slope of the line in the diagram above) that effects the coordination of the traffic signals. It is these physical constraints that don’t allow traffic to be coordinated perfectly in both directions.
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