Exploring the tech of traffic lights

Love ‘em or loathe ‘em, traffic lights are a common feature on our roads and a major part of our everyday journeys, playing a vital role in terms of safety and traffic flow. Although these ubiquitous traffic controls have been around since the 1920s, the advanced technology behind today’s traffic lights is going through a ‘Smart’ revolution, with the introduction of new technology aimed at managing flow and reducing queues as much as possible. Rob Perry, design engineer and all-round traffic light expert from Dorset Council, explains.

Traffic lights have been around for almost a century. The first electric traffic lights in the UK were installed in London in 1927, although an earlier mechanical lever design, which required a policeman to operate it, was experimented with in 1868. The Convention on the Unification of Road Signals, which was signed in Geneva in March 1931, gave traffic lights the familiar ‘red, amber, green’ signal sequence which is pretty much universal across the globe today.

All road authorities in the UK, including Dorset Council and BCP Council, now design traffic light junctions and pedestrian crossings to a specific set of national standards. This means that no matter where you travel in the UK, the traffic signals and junctions work and look the same – including their operation, signal heads, pole placement and the white lining on the road.

Modern signal heads (the light-display part of the traffic signals) now use LEDs to ensure junctions are highly energy efficient and low maintenance.

As well as the white lines and signs, you will usually see fine black lines in the carriageway on the approach to a set of traffic lights. These are slots cut into the road surface that contain sensors that detect the traffic flow.

There are other sensors on the traffic light poles above the signal heads which detect vehicles, pedestrians and cyclists on the crossings and approaching the junctions. These high-tech sensors can use a variety of detection methods including induction, microwaves, radar, infra-red, vision (ordinary cameras), magnetometers or good old-fashioned push-button controls.

Information from these sensors is fed to the sophisticated junction control computer, which controls the lights in a sequence (known as the ‘cycle time’) and allows each approach road and pedestrian crossing to display a green signal in turn. The maximum cycle time is generally fixed, but cycle time will vary in length dependent upon the level of demand at that time. It should be shorter at quieter times.

These controllers use software with complex algorithms to read the sensor data and learn about the patterns of traffic flow at the junction. The computer can then determine which roads and crossings have the most demand and will proportionately share out the ‘green light time’ available. In the near future, the junction control computers are likely to use Artificial Intelligence to further improve their operation.

Similar sensor technology is used in temporary traffic lights, but the computer control is not as sophisticated.

The control computer will allocate more ‘green time’ to the roads with the highest traffic demand in order to keep the busiest routes flowing. When demand exceeds the capacity of the junction, queues can form, but the computer will aim to share out the delay based upon the priority of the road – a quiet side road will get a shorter ‘green time’ compared to the busy main road.

The computer can also jump parts of the cycle sequence, such as a pedestrian crossing or a side road if there is no one waiting to use a facility.

Many junctions also ‘rest’ when the roads are empty, showing a red light on all arms of the junction. When a vehicle approaches, or a pedestrian presses a push button, the sensors leap into action and the control computer can quickly change the lights to green to let the vehicle through or the pedestrian cross safely. In more dense urban areas where junctions are close together, they can operate as a network or series of signals which are linked to allow the traffic to flow better from one set of lights to the next.

Faults and problems are reported automatically by the system to engineering centres within Dorset Council and BCP Council, making it quicker for an engineer to attend and fix the problems. Members of the public can report problems using an online form (see links below) or by looking for the phone number on the control cabinet next to the lights.

Report traffic light faults: BCP Council

Report traffic light faults: Dorset Council

Technology will continue to evolve to make traffic lights as effective as possible, but there’s definitely more to them than meets the eye! They play a key role in keeping all road users safe and the infrastructure running smoothly, so make sure you pay attention, even if it seems like there’s no reason for a red light.