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Traffic Lights At Junctions

Traffic light timers

Before discussing traffic light sensors, it helps to know about traffic light timers.

Traffic lights are normally controlled by timers. These timers change the traffic lights at set times during standard traffic flow conditions. At a crossroad junction for example, the timing could be set for the main road to have 30 seconds of green, then the side road to have 20 seconds of green, then back to the main road, and so on. At certain times of the day, and usually during certain times of the night, the timers are changed to different times. As an example, during certain night time hours the side road at a junction might be set for the traffic light to remain on red all night except for when a vehicle approaches, and that's one example of where the traffic light sensor comes in (explained later in this page).


Traffic light sensors

Traffic lights at junctions usually have sensors that assist with the flow of traffic. These sensors are usually located:-

1. directly above the traffic lights;
2. or, buried within the road on approach to the traffic lights and/or where the first vehicle would normally wait if the traffic light is on red;
3. or, a combination of both sensor types.

The sensors normally located directly above the traffic lights

These sensors detect the body of a vehicle approaching the traffic lights, on some junctions the traffic lights will not change to green until a vehicle is detected approaching them (they cannot detect light, so for those who flash their headlights at roadworks traffic lights to encourage them to change, it does not work, if light did cause the traffic lights to change then the traffic lights would be constantly changing during daylight hours, causing mayhem). Examples of how these sensors control the timing of the traffic lights are as follows.

1. When a vehicle approaches, the traffic lights will either change to green on approach or change to green at the next timed allocation;
2. The traffic lights will remain on red until a vehicle approaches;
3. A few more seconds will be added to the green if a higher volume of traffic is detected to be approaching;
4. When the green comes on, time will be deducted from the green (this could mean down to as little as a couple of seconds) if a lower volume or no traffic is approaching.

The timer settings and the sensor controls for each junction are set according to the traffic studied at that junction and the layout of the junction.


Unfortunately there is an issue with this type of sensor when there is a traffic queue waiting while the traffic light is on red. The issue arises if the first vehicle in the queue attempts to move off when the green comes on but they stall. Some traffic light systems are programmed to almost immediately change back to red if there is no traffic detected to be moving. Now the real issue comes in. As mentioned earlier, some systems are programmed to remain on red until a vehicle approaches. Due to the traffic being queued up at the time the traffic light changed to red, there is no traffic moving towards the traffic light, or the sensor, therefore on some systems the traffic light will remain on red and not change back to green. A very difficult situation to be in.

A similar issue arises when the traffic moves off on the green but a vehicle in the queue takes time to move off, thus leaving a longer than normal gap between that vehicle and the one in front. The sensor detects the gap as an indication that no more traffic is approaching and thus changes the traffic light back to red. This will mean fewer vehicles being able to continue and they will have to wait until the next green.


The sensors buried into the road

These sensors can usually be seen in the road as a line creating an approximate one metre square or as lines looking like a large hash marking. These sensors detect pressure i.e. they detect when a vehicle is on them.

These sensors change the traffic lights just like the sensors above the traffic lights, but with a few added features and a significant difference. Examples of the extra features are as follows.

1. At some junctions, there is a sensor buried in the road out at the point where a vehicle would wait for a gap in the traffic before turning right. This sensor is usually for controlling the filter arrow. Examples of theses controls are:-
2. If there is a queue waiting to turn right, then the green arrow will come on allowing the traffic to turn;
3. If there is a long queue waiting to turn right, then a few seconds will be added to the green arrow, allowing more vehicles to turn;
4. If only a couple of vehicles are waiting to turn right (and they are out in the middle of the junction waiting to turn), then sometimes the green arrow is not turned on. This is because the couple of vehicles should turn right as soon as they have a gap in the traffic, or turn right when a gap is created by the traffic light turning to red for the oncoming traffic. This allows for better traffic flow.
5. At some junctions, traffic can only turn right if the green arrow comes on (there is no green circle). If a sensor is buried in the road just before the wait line at the traffic lights then some green arrows will not come on until a vehicle has driven onto the sensor. So if you approach and the traffic light is on red, make sure your vehicle is on the sensor or you'll be waiting there all day.

So what's the significant difference with these sensors buried within the road. These sensors have the same issue for when the first vehicle stalls when attempting to move off. The difference though is that when the traffic light changes back to red, the system will recheck the pressure sensor and if a vehicle is positioned on the sensor then the traffic light will change at the next designated time.

At some traffic light junctions, both types of sensor are used in combination to assist with the control of traffic flow.

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