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Page last updated at 06:09 GMT, Friday, 17 April 2009 07:09 UK
Welcome to the red light district

By Tom Colls
Today programme

Traffic light
Traffic light timings calculated using complex computer simulations

There's nothing more frustrating than sitting at a red light, watching smiling drivers on the other side of the road speed past.

But before you sound your horn and shout at the lights, consider the enormously complex network of roads, lights and junctions in which the single vehicle is just a tiny part.

In London alone, an average 11 million car and motorcycle journeys take place every day. The city has 6,000 traffic lights controlling the flow and a hi-tech control centre monitoring its success.

It is a far cry from the first traffic signals - which were erected outside the Houses of Parliament in 1868 and which controlled the traffic using semaphore arms.


As complex as the traffic network might seem, the technology on which it depends is surprisingly old fashioned.

Patent drawing for Garrett A. Morgan's traffic signal
A 1923 traffic signal - one of the first that could be operated remotely

Two main systems operate in the UK. The first uses data collected on traffic flow in a certain area to calculate timings for the lights, running the red lights to a strict timetable.

The basic maths of the system was developed by researchers FV Webster and BM Cobbe in the 1960s and is still used to this day - although refined somewhat by the advent of computer simulations.

The other method is a little more hi-tech.

Coiled under the tarmac at junctions and crossroads across the UK, are loops of cable which act as metal detectors as cars pass over them.

This information is passed to a central computer, which uses complex algorithms to calculate the optimal time each light should spend on each colour, ideally creating a smooth flow of traffic throughout the network.

The computers make small adjustments to the timings of the lights, slowly iterating towards the best possible traffic flow.

The most hi-tech development is the use of transmitters in buses, which are tracked by the system so that bus passengers are guided as quickly as possible to their destination with the minimum disruption to other traffic.

And in case you were wondering, some lights have a sensor on the top, with the lights programmed not to change at quiet times unless traffic is spotted - so it sometimes is worth edging carefully forward.



John Tenten explains how the London Traffic Control Centre keeps its eyes on the capital's traffic lights


Mathematical algorithms, and futuristic operations centres can only get you so far. There is a fatal weakness in the smooth running of city centre traffic - cars are driven by people.

"Drivers are free agents," explains Professor Michael Bell, Professor of Transport Operations at Imperial College, London.

UTC Control Centre - The central hub from where Urban Traffic Control is co-ordinated.
Scoot - Split Cycle time and Offset Optimisation Technique, the system used for real-time traffic control
Selective Vehicle Detection - Sensors in roads which pick up the movements of certain vehicles fitted with a special tag
Fixed Time Plan - Automated method of controlling traffic lights to pre-programmed timings

"There is no way of getting down to a neat mathematical formula.

"You can optimise signals mathematically but then your calculations could be instantly invalidated by drivers deciding to take another route.

"There are just signals adjusting to the drivers, adjusting to the signals, adjusting to the drivers, adjusting to the signals..."

What sometimes determines how long you see green or red are not computer systems, but politicians.

"You might find it easier to get out of a city than get into a city," says Mark Crabtree, principle engineer at the Transport Research Laboratory (TRL).

"That is a deliberate policy to control how much traffic can get into the city."

A policy known as "gating" where traffic is purposefully made to wait to enter town centres is common. But the schemes are a "necessary evil" according to Mr Crabtree - if traffic coming into a city were allowed a free rein, the centre would quickly find itself in gridlock.

But as Professor Bell says, the idea that the traffic controllers are out to make driving as uncomfortable as possible is far from the truth.

"I don't think any designer of signal control systems would have gone out of their way to increase stop-go driving," he says.

"The only way you could stop drivers being stopped at traffic lights would be to take them out of the ground."

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