Shropshire Council


On this page you'll find the answers to some of the questions we regularly get asked, which may help you understand the traffic signal process better.

How do traffic lights work?

Vehicle sensors are sited on each approach to the signals. These detect vehicles as they pass them, and either change the signals to green or extend the green signal for that approach. At busy times the green period will extend up to a pre-set maximum time, and the signals will follow a set sequence. At other times the signals will respond to traffic as and when it arrives at the junction.

How are traffic lights controlled?

Traffic lights can be controlled in different ways depending on the location, day of the week or time of day.

How do traffic signals detect vehicles?

Vehicles can be detected in several ways. Often loop detectors are used, and these are buried in the carriageway on the approach to the signals or crossing. Loop detectors sense when a large metallic object such as a car engine passes over them. More recently the method of loop detection has been upgraded to magnetometer pods, which operate in a similar manner to loops, but use radio signals to communicate detection back to the signal controller.

Another method uses above-ground detection mounted above the signals to sense the movement of approaching vehicles.

We're currently looking at new technological methods of vehicle detection which use special camera detection. This apparatus doesn't record vehicle movements, but uses the movement of vehicles into and out of designated areas as a counting mechanism.

Why don’t the lights respond to my bicycle/motorbike?

Below-ground detectors are able to be tuned to effectively recognise smaller vehicles such as bicycles and motorbikes. It could be that there's a fault present, such as a damaged loop or a power pack that needs changing. If you suspect that this is a fault, you can report it online.

Traffic lights change when there is no traffic there. Is this normal and why does it happen?

There are several reasons why the traffic signals can change in the absence of traffic. It could be as a result of a fault with the equipment, but some traffic signal junctions are configured to operate in this manner, and provide fixed amount of green time to each traffic phase. This is usually in order to aid progression for traffic where several sets of signals are located closely together.

Why doesn’t the right-turn green arrow always appear when I’m turning right?

This is usually because the appearance of the right-turn arrow is dependent on the number of vehicles waiting to turn right. In order to avoid unnecessary delay, junctions are often configured to allow the first two or so vehicles to turn right during the period when the oncoming traffic loses right of way and before the next traffic phase begins. When more than two vehicles are waiting to turn right they will require more time to safely clear through the junction. The green right-turn arrow then appears to provide additional time for this movement.

Can there ever be green signals at the same time for two conflicting traffic movements?

In general terms, and where a green signal indicates a specific priority such as a right-turn arrow, no. Under these circumstances it's not possible for two conflicting approaches to be green at the same time. In the event that this condition is detected, the signals would automatically switch off in less than a second. This is a requirement of the regulations applying to traffic signals in the UK. This is the same if a pedestrian phase and a conflicting traffic phase were to go green at the same time. All the equipment is thoroughly tested and checked to ensure that green conflicts don’t occur.

Where it's permissible for vehicles to turn right, across approaching vehicles, and there isn’t a right-turn arrow present, then technically there can be a conflict and drivers need to ensure that they have sufficient clearance to enable them to cross approaching traffic safely before proceeding.

Can signals interfere with my TV/radio reception?

No. Traffic signal equipment operates within a very limited frequency range, which is different from that used for TV or radio.