MOTORISTS across York were hit by traffic chaos after a computer failure threw the city's traffic light network into disarray.

There were major jams on roads leading in and out of York yesterday afternoon, with many journeys taking twice as long as usual. Traffic was backed up along large stretches of the inner ring road, with the area between Lawrence Street and Monkgate badly hit. While each individual set of lights was functioning properly, the system which manages the network as a whole failed.

The timing for the different sets of lights around the city is usually staggered to optimise the flow of traffic, but yesterday's failure meant the system broke down.

Peter Evely, head of network management for the City of York council, said: "The urban traffic management control (UTMC) system we use controls a whole host of traffic signals, and we manage the whole network simultaneously. That way, we effectively create road-space that is not there. But when it goes wrong, that extra space disappears and you get queues all over the place."

Normally, the council's main computer in St Leonard's Place sends a message to the BT switchboard exchange in Stonebow, from where a signal gets sent to the appropriate traffic light telling it to change.

Yesterday, the BT switchboard failed, which meant the lights reverted to the back-up system, and operated independently of each other, preventing a steady flow of traffic.

"All the lights were fine, and were working individually. It was a synchronisation problem," Mr Evely said.

"This kind of problem is incredibly rare. It certainly wasn't the first thing we thought of when the problems started appearing."

York resident, Andy Clarke, 23, was caught in the chaos as he drove home from work in Bishopthorpe to Haxby Road. He said: "The journey that normally takes me 20 minutes took 40 instead. Around Walmgate Bar and Monkgate it was chaos."

Kathryn Simpson, who was stuck at a set of lights in Clifton, said: "They kept changing every second."

The city council said the fault was rectified at about 8am today.

Updated: 10:28 Thursday, April 21, 2005