Anatomy of a tailback

Small actions cause big jams. Jeremy Hart analyses the latest research on motorway habits

IT WAS one of those mysterious journeys, like something out of The Twilight Zone. All the way down the M6 from Cheshire to London, the Bank Holiday traffic crawled along in eerie silence, bumper to bumper, while the opposite carriageway was deserted. For brief periods the log- jam cleared and we crept up to 60mph, but for the most part we dawdled. We waited for flashing blue lights on the hard shoulder, the twisted metal of a serious smash, diverted traffic filtering in from a sliproad ahead. By Coventry, none of these things had appeared. No cones, no roadworks, no tell-tale carpets of fragmented glass. After a day on the road we limped into London, wondering about the spectral causes of this jam.

Researchers as far apart as Berkshire and California have been wondering much the same thing. Roadworks and accidents alone do not explain the severe delays on multi-lane roads. When a motorway is busy, it seems, the tiniest driving irregularity - such as gentle braking, or rubber-necking - can cause delays. Research in the US has shown that even a driver parked by an emergency telephone, or a workman mowing the grass verge, is enough to make traffic grind to a halt several miles away.

In America, drivers spend 2.7 billion hours a year sitting in jams. Not surprisingly, scientists there are accelerating their efforts to find a solution. What they have discovered is that superhighways - like the proposed 14-lane stretch of the M25 west of London, scrapped by the Government two weeks ago after vociferous local protests - are not the answer. Wider is not necessarily better. Even a 12-lane freeway can snarl up in minutes. The reason? Jerky driving.

British researchers are also familiar with the phenomenon. "If the motorway is full to capacity, one driver braking can cause a shockwave that means someone further back has to stop," says Peter Still, a traffic flow expert at the Transport Research Laboratory in Berkshire. "Often this happens because the speeds in the lanes vary considerably." If speed is kept uniform in all lanes, the aptly named Mr Still explains, the need to jump from one lane to another is reduced. "There is then no benefit in driving in the `fast' lane. Also, when vehicles are moving at smaller differential speeds, the effect of the shockwaves is less dramatic."

With this in mind, a new traffic regulating system was introduced in Britain at the beginning of this month. Drivers on the south-west quadrant of the M25 can no longer assume that the legal speed limit will be 70mph. In the heaviest traffic it drops to (and is enforced at) 40mph. When conditions are freer, the national limit is reinstated. By means of roadside sensors linked to high-intensity signs, the warning is flashed to drivers: they must slow down - or snarl up.

But speed limits flashed on dot-matrix screens are notorious for being ignored. What makes these any different? The answer is that Gatso speed cameras are linked to the system. The police are confident that drivers will take as much notice of the M25 variable speed limits as they do in built-up areas where cameras have become an effective deterrent.

Sergeant Roger Reynolds, of the Metropolitan Police traffic camera unit, is convinced the system will work. "The first road to get these cameras was the A316, off the M3 into west London. Before, the number of offenders over 60mph (in a 40mph limit) was 23,993 in a 22-day period. Within a month of introducing the cameras, it was down to just 100 a week."

An unexpected benefit of the speed restrictions was that drivers found they were getting to work on time. "They wrote in saying there were fewer jams and that their journey times had been cut," says Reynolds. On the teeming Cromwell Road in London, in fact, sticking to a steady 30mph will actually give drivers a clear run through all the green lights. "Going faster will only mean you have to stop and start and get stressed," Reynolds explains.

Sound advice this may be, but in much of the capital a top speed of 30mph is something drivers can only dream about. At 11mph, the current average speed of traffic has reached an all-time low - slower than Victorian horse- drawn transport. Even in the small hours, when the streets are empty, the best (legal) speed achieved is 20mph. Ironically in view of Sgt Reynolds' remarks, traffic lights are to blame.

On average, road commuters in Britain are delayed for eight days each per year. The cost to industry has accelerated the Government's efforts to find a solution. John MacGregor, the former Transport Secretary, convinced road-widening was the answer, allocated £250m to widening the country's worst motorway bottleneck, the Surrey section of the M25. His successor, Dr Brian Mawhinney, is still committed to road-widening in spite of recent setbacks.

The Californians have come up with a cheaper and less environmentally damaging way of freeing up the existing road network. They, too, are regulating traffic flow using electronic sensors - but with a difference. They do not impose lower speed limits, they divert traffic to less jammed freeways (such a road-hopping technique could not work in Britain, however, as there are so few parallel motorways).

The discovery that road-hopping is more effective than road-widening came as a result of the Los Angeles earthquake. Engineers given the task of rebuilding the city's devastated freeways were able to build wider roads. What they found, though, was that the new, improved freeways drew more motorists - and resulted in just as much congestion. Professors Michael Cassidy and Gordon Newell at the University of California deduced that the size of the freeway actually mattered very little. And while electronic traffic diversion might help, it was the performance of the drivers on the roads that sabotaged the system's fluidity and efficiency.

Just as erratic lane-changing causes delays on British roads, so the American habit of "undertaking'' causes bunching on freeways. Drivers can pass to the left or the right of other vehicles, so there is no "fast" lane. But the more uniform speed means faster drivers still have to weave in and out to pass slower traffic. Motorists who are forced to brake to allow a faster car through then instigate a knock-on braking wave.

"Allowing undertaking wouldn't improve things here," Sgt Reynolds says. "The British public don't have the lane discipline - and that is still the major cause of bunching." In the end, no traffic system will over- ride human nature, particularly curiosity. "Some things you just can't change," Sgt Reynolds says, "and rubbernecking is the classic. If there is an accident, everyone will slow down to see if there are any dead bodies. Drivers are only human." !