Los Angeles traffic jam
While the underlying cause of a jam might be an accident, a bottleneck, or drivers simply changing lanes on busy roads, it is how the drivers react in the cars behind that causes traffic to slow to a halt.
Researchers say aggressive motorists, who drive too fast and too close to the vehicle in front, or timid motorists, who leave too big a gap, send a “wave of deceleration” backwards down the road until traffic grinds to a stop.
Such behaviour leads to the stop-start traffic jams which infuriate motorists.
Since 2001 the number of cars on Britain’s roads has risen considerably from 24.6 million to 31 million, leading to more traffic jams.
A report earlier this year estimated that by 2025, drivers in Britain could be wasting 656 million hours a year – the equivalent of 75,000 years – sitting in traffic jams.
Dr Jorge Laval, from the school of civil and environmental engineering at Georgia Institute of Technology in Atlanta, who conducted the research into aggressive and timid drivers said it was important to understand what causes the jams.
“Stop and go driving is a nuisance for motorists throughout the world. Not only does it increase fuel consumption and emissions, but it also imposes safety hazards,” he said.
Dr Laval, who worked with colleagues at the University of Lyon in France, found that when drivers changed their speed, they caused drivers further back to change their speed. The change in speed passed like a wave, backwards through the traffic.
If all the drivers behaved the same way, then traffic would not come to a halt. But the researchers found that in real life situations, the behaviour of aggressive and timid drivers led to the slowdown getting worse.
The scientists, whose research appears in a special edition of the scientific journal Philosophical Transactions of the Royal Society, found that timid drivers had the biggest impact because they “shied away” when the car in front started slowing down, and deliberately started driving even more slowly to increase the gap between them.
This then led to cars further behind going more slowly.
Aggressive drivers also caused speed to drop because they braked hard at the last moment to avoid driving into the car in front. They then had to drive more slowly to open up a space again.
Dr Eddie Wilson, an expert on traffic modelling at Bristol University, said: “The exact point at which a traffic jam starts is very difficult to measure as it often causes a wave that produces a jam 20 miles further back.
“My own feeling is that it has more to do with lane changes, particularly at junctions when there are large numbers of vehicles changing lanes.”
In light of the unpredictable human behaviour on roads, researchers from Cornell University in Ithaca, suggest that robot driven vehicles could be used to ease traffic jams, particularly in urban environments.
They say that vehicles that are capable of sensing their surrounding environment, and their proximity to other cars could maintain safe distances while helping to keep traffic moving.
Car manufacturers including Toyota, General Motors and Mercedes, are already developing vehicles that use computers to either take over control from motorists in dangerous situations or are completely robot controlled.