Traffic Signals Information
The first traffic signal in Queensland was switched on, 21 January 1936. It was located at the intersection of Ann, Upper Albert and Roma Streets.
Queensland now has around 1,700 signalised intersections under the central control of a fully integrated Intelligent Transport System (ITS) known as STREAMS.
The Department of Transport and Main Roads uses STREAMS to:
- coordinate traffic signals;
- record faults (eg. blown traffic signal lamps);
- record data for performance monitoring (eg. traffic volumes at intersections);
- manage traffic incidents and special events;
- provide traveller information; and
- provide parking guidance.
Why have Traffic Signals
How do Traffic Signals Work
Modes of Operation
Pedestrian Crossings at Traffic Signals
Roundabouts vs Traffic Signals
Reporting Problems / Concerns
Traffic Signal Coordination
Why have traffic signals?
The main reason for traffic signals are:
- to allow road users to safely navigate through an intersection;
- to give priority to a particular direction / mode of travel at different times of the day; and
- through coordination, allow large volumes of traffic to pass through the network with minimal delay.
How do traffic signals work?
A standard set of traffic signals consists of:
- a traffic signal controller;
- vehicle detector loops and pedestrian push buttons;
- traffic signal lanterns; and
- posts, pits and underground electrical cables that connect all the components together.
The traffic signal controller
Housed in a grey metal box on a corner of the intersection, the controller is the 'brain' of the system. It is a computer that processes information received from the detector loops and pedestrian push buttons and changes the signal lanterns in accordance with its programming. Based upon the prevailing demands, the controller determines the length of the green signal for each traffic movement and controls the transition from one combination of green and red signals (known as phase) to the next. It can operate in a 'stand alone' manner or be programmed to coordinate with a series of adjacent traffic signals.
Vehicle loop detectors and pedestrian push buttons
Vehicle loop detectors and pedestrian push buttons are the 'eyes' of the system. They are mechanisms motorists and pedestrians use to make the controllers 'see' them and change the signal to give them right of way.
Vehicle loop detectors are loops of wire buried in the road leading up to the stop line at the intersection. When a vehicle is passing over the loop the magnetic field (inductance) of the loop changes. The controller detects that a vehicle is waiting to proceed through the intersection.
Likewise, when the pedestrian push button is pressed the controller knows that a pedestrian is waiting to cross.
Traffic signal lanterns
Traffic signal lanterns are the means by which the controller directs traffic. They tell the road users when to go and when to stop. Queensland traffic signal lanterns follow universal traffic signal colour conventions. GREEN = Go if it is safe to do so; YELLOW = Stop if it is safe to do so; and RED = Stop.
Modern pedestrian signals use the symbolic walking green and standing red figures figures although there are still older lanterns around that display WALK and DON"T WALK.
Over time, the department is converting standard signal lanterns to LED (Light Emitting Diode) lanterns which are very energy efficient and long lasting.
Signal phases and cycles
Each combination of green and red signals that the controller is programmed to display is called a phase. Each phase has a programmed minimum time so that once the signals have entered a phase they cannot change again until the minimum time has expired. One complete sequence of all the vehicle and pedestrian movements (phases) at an intersection is known as the signal cycle. In Queensland, the duration of a cycle is usually between 50-150 seconds. The cycle time varies by location and time of day.
The yellow signal
Traffic signals change from green to yellow to warn approaching motorists that the signal is about to turn red. The length of the yellow signal depends on the speed limit of the road. Most urban intersections have a yellow time of four to six seconds. The yellow signal means stop if it is safe to do so. Any vehicle travelling at the speed limit toward a green signal that changes to yellow should have sufficient time to stop safely or clear the intersection before the signal changes to red if the driver has entered the intersection.
The all-red time is the time between the end of the yellow signal on one phase and the commencement of the green signal on the next phase. All-red time is used to provide a safe clearance for vehicles that cross the stop line towards the end of the yellow signal as they may be in danger of colliding with vehicles or pedestrians starting in the following phase.
The all-red time is based upon the physical size of the intersections and speed limit of the road. Similar to the length of the yellow signal, the all-red time does not change throughout the day.
Modes of operation
The traffic signals at each intersection can be programmed to operate in an isolated mode or be coordinated with traffic signals at adjacent intersections to allow the progression of traffic along the road.
In isolated mode, traffic signals changes are driven by the vehicle loop detectors and pedestrian push buttons at the intersection (see above). Isolated mode works very well for intersections with low volumes of traffic, no major flow of traffic in one direction or intersections that are a long way from each other.
The other mode is 'coordinated'. For traffic signals to be coordinated they need two things:
- A common signal cycle time: The signal time is the time it takes to run through one complete sequence of all the vehicle and pedestrian movements (phases) at an intersection; and
- A timing offset between the start of one intersection's main green movement and the next intersections main green movement so that vehicles travelling at the designated speed limit leave the first intersection on the green signal and reach the second intersection at the same time as its signal turns green.
The benefit of coordinating signals is that large volumes of traffic can pass through multiple signals with minimum delay.
The disadvantage is that because the common cycle time is set to meet the needs of the largest and most complex intersection in a series, signals at smaller intersections in the series can appear to change too slowly.
When coordinating traffic signals, the department works hard to strike balance between allowing the progress of vehicles along the main road and keeping the wait times for side street vehicles to a minimum.
Pedestrian crossings at traffic signal
It is a common misconception that people have to get all the way across a 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 signal that is timed so that a pedestrian who leaves during the walk will have enough time to safely cross the roadway.
Remember, if a pedestrian leaves the footpath at the very end of the WALK signal, they will have enough time to safely cross the road before the next movement of traffic begins.
Some pedestrian push buttons also have an audio and tactile (touch) component to them. These are the push buttons -called audio tactile push buttons - that emit a beep beep sound. Visually impaired people can use the steady beep beep sound (homing signal) to locate the push buttons. When it is time to cross the road the push buttons produce a series of fast beeps.
The face plate of the audio tactile push buttons also physically pulse in time with the audio sounds produced providing the tactile part of the push button.
Roundabouts vs traffic signals
Traffic engineers use the Australian Standard AS1742 and Austroads Design Guidelines to decide which option - the installation of signals or the construction of a roundabout - will best suit a particular intersection.
Some of the pros and cons are:
- are less expensive than roundabouts to install but higher maintenance costs;
- are normally only considered for intersections with high volumes of traffic;
- use less space than roundabouts, and
- are safer for pedestrians and cyclists
- are more expensive than traffic signals to install as they usually require land resumption but they are cheaper to maintain;
- can make traffic flow more smoothly when traffic volumes are low to medium; and
- have built-in priority rule, which means that heavy traffic in one direction can tend to dominate and cause excessive delays to traffic in other directions.
In deciding between traffic signals and a roundabout, traffic engineers will assess each intersection, taking into account factors such as traffic volumes, available land, needs for pedestrians and cyclists, the intersection's accident history and the intersection's overall place in the transport network.
Reporting problems / concerns
Traffic signal faults can be reported by phoning the Traffic Signals Fault Line on 13 19 40.
When calling the fault line to report a fault or raising a concern it will assist the operator if you can supply the following information:
- The location of the traffic signals - eg. the intersection of Finucane Road, Shore Street and Delancey Street at Cleveland.
- Details of the fault/concern -eg. The green arrow to turn right from Finucane Road travelling east into Delancey Street is not working.
- At what time of day did you notice the fault? - eg. I noticed the fault on my way to work at 7.00am this morning
- Any other information that you consider relevant.
Traffic Signal Coordination
The following example shows the coordination of three made up intersections, Smith Street, Jack Street and Bill Street. The left hand side of the diagram shows the actual intersections on the street. The right hand side of the diagram shows what is known as the space-time diagram. The x-axis (horizontal) represents time and the y-axis (vertical) represents distance along the roadway. At each intersection, going across the page is the green and red signals (A phases and B phases) that the drivers would see over time.
As shown in Figure 1 below, if the car leaves Smith Street at point A it would get to Jack Street (point B) as the signal turns green. Likewise the signal would turn green as the car reaches point C at Bill Street. In this case the timing of the signals is really good for the car heading north as the offsets between the start of Smith St's green and the other intersections allows for good northbound coordination. Click here to see an animation | animation | transcript.
Figure 1 -Optimised traffic signals heading northbound
Now using the exact same settings (offsets, cycle and phase lengths) let's look at a car heading southbound. In Figure 2 below, if the car leaves Bill Street (Point D) at the start of the green it will get to Jack Street (point E) mid way through the green and hit the red at Smith Street (point F). Click here to see an animation | animation | transcript.
Figure 2 -Non optimised traffic signals heading southbound
The same coordination offsets that provides great coordination in the northbound direction resulted in poor coordination for the southbound direction. This is shown in Figure 3 below.
Figure 3 - North bound and south bound coordination
The above example shows that it is distance between the intersections (and speed limit of the road) that determines the possible coordination. When working out the coordination for the traffic signals we take into account the volume of traffic heading in each direction and try to provide favourable coordination for the direction with the greater volume.
Coordination of signalised intersections is achieved through providing green band progression to vehicle platoons travelling along a corridor. In coordinating traffic signals we assume that a vehicle's movement along a corridor is known and can be anticipated. Knowing when a vehicle is going to pass an intersection allows for a green light to be pre-programmed for that vehicle's direction of travel. In an ideal world, once a vehicle has joined a platoon of vehicles it should not stop at a red light as long as it stays travelling with the platoon. However, in reality, numerous factors alter the progression of a vehicle platoon including:
- How far the platoon of vehicles has been travelling for (platoons of vehicles tend to space out the further they travel);
- The vehicles in a platoon may not all travel as the same speed;
- The drivers in a platoon may react differently to various road conditions and accelerate and decelerate at different rates;
- Different position of a vehicle in a platoon might experience different progression depending on green time of upstream and downstream intersections. In the examples above if the amount of green time for the main street (A phase) at Jack Street had to compete with a large side street demand (B phase) not all vehicles in the platoon would get through. This is demonstrated in Figure 4 below; and
- Larger, more complex intersections require a greater number of phases to service all demands. With the increase in the number of phases the amount of green time for the main coordinating phase (A phase) is reduced. This means the width of green band available for the main street is shorter. This is demonstrated in Figure 5 below.
Figure 4 - Heavy side street traffic limiting the green time available for the Main Phase.
Figure 5 - Complex phasing limiting the green time available for the Main Phase.