Highway Maintenance

Traffic Lights - Frequently asked questions

How do traffic lights work?
Vehicle sensors buried in the carriageway on each approach to the signals, detect vehicles as they pass over 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 do the lights detect vehicles?
Vehicles can be detected in two 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. Alternatively above ground detection mounted above the signals may be used which sense the movement of approaching vehicles. Pneumatic pads/sensors were used many years ago but their use has long since been made obsolete by loop detectors and above ground detection.

Why don’t the lights respond to my bicycle/motorbike?
They should, the fact that they are not suggests a fault. Inductive loop detectors have a sensitivity setting which can be adjusted to detect smaller vehicles. In the case of above ground detection it is possible that your approach speed is not high enough. In these cases, special low speed detectors can be installed.

Traffic lights change when there is no traffic there. Is this normal? Why does it happen?
There are several reasons why the traffic signals can change in the absence of traffic. If from experience the junction normally operates differently, then it is likely to be occurring as a result of a fault with the equipment. However, 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.

Why is the traffic light on the off-side of the road not facing down the road towards on-coming traffic?
The signal on the off-side of the road (referred to as the secondary signal) is directed slightly towards the vehicles waiting at the stop line. Drivers of vans and lorries have a problem seeing the nearside signal when stopped at the stop line. The nearside signal is the important one for drivers approaching the signals as the off-side one can be obscured by traffic travelling in the opposite direction.

Can there ever be green signals at the same time for two conflicting traffic movements?
No. It is not possible for two conflicting approaches to be green at the same time. in the event that this was detected, the signals would automatically switch off within 500 milliseconds. 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 get green at the same time. All the equipment is thoroughly tested and checked to ensure that green conflicts don’t occur.

Are signals preferred to roundabouts?
There are many possible reasons why traffic signals have been installed instead of a roundabout. The most common are:

  • roundabouts only work well if flows are balanced between all arms
  • they are generally much more expensive to construct than signals and
  • they require more land which isn’t always available, and
  • it is much harder to provide controlled crossing points, access for pedestrians and other vulnerable road users, within the geometric constraints of a roundabout.

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. The Radio communications Agency may be able to help if you are experiencing problems with interference.