and ROB CRANE
For drivers across Britain this Easter, it will be a familiar scenario: sit in a jam for hours, only to find, when the traffic begins to move again, that there was no apparent reason for it.
Now, a super-computer produced by a team of scientists in Edinburgh has shown what causes inexplicable hold-ups: drivers who sit in the middle lane, and those who follow too closely behind the car in front, and have to brake abruptly.
"You'd be surprised at what people slow down for," said Mark Smith, marketing director for Quadstone, a spin-off company formed by the Edinburgh team. He said an aeroplane passing overhead, or an accident on the other side of the road, can lead to a "shockwave" of braking, which travels back through the traffic queue, as drivers react to brake lights in front by braking themselves at up to 12 miles per hour. This means that five minutes after the event occurred, people a mile away are still slowing down.
When the traffic is heavy, that quickly leads to "flow breakdown", wherever the number of cars on the road is at a critical level - around a hundred cars passing any point every minute. The result: a traffic jam appears from nowhere. Bad lane discipline, such as middle lane hogging, makes it more likely, by reducing the road's capacity.
"On the M25, we found that in the four-lane section, when all are properly used, you can get 8,000 vehicles past a point in an hour," said Dr Smith. "When you get everyone bunched into the outside lane, because there's an empty inside lane, a lorry in the second lane, and a car travelling at the same speed as the lorry in the third lane, the flow falls to 2,500 vehicles."
Their work, in a project called Paramics, at the university's Parallel Computing Centre, may lead to "traffic forecasts" that enable drivers to find out which roads will be busy, based on the traffic which is already on them.
Those setting off for Easter breaks may have been grateful for forecasts yesterday as big jams built up, with routes out of many big cities described as "a nightmare".
Motorists have been warned to expect long delays over the weekend as holidaymakers are joined on the roads by people tempted out by good weather.
A spokesman for the RAC said yesterday: "There are certain points around the country where it really is horrendous. We're looking at some pretty bad delays." Routes out of London and in the West Midlands were particularly busy, as were all roads leading to airports and ports.
And an AA spokesman warned drivers to plan journeys to avoid bottlenecks. "It looks like the weather is going to be great, so the coast and many Easter events could well attract record crowds."
Forecasters predict it will remain sunny until tomorrow when it will become cloudier, although it should stay dry. About 1.5 million Britons will spend the holiday abroad, with Heathrow yesterday handling 165,000 passengers. In all, 2 million passengers will pass through the airport by 13 April.
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