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Driving in the rain

New Scientist

RAINY weather influences road deaths in a far more complicated way than previously thought. Rain does not increase the number of deaths in crashes, provided it rained the previous day. But a heavy shower after a dry spell causes a lethal surge in accidents. Daniel Eisenberg of the University of California, Berkeley, stumbled across the link while studying the dangers of driving under the influence of drugs. He knew that there are more accidents on rainy days and wanted to show that this factor wasn't influencing his results.

But when Eisenberg calculated the annual and monthly figures for rainfall taken from the US Weather Service's 20,000 weather stations, and compared it with nearly 430,000fatal crashes between 1975 and 2000, he found that the number of deaths fell in rainy months. It was only when he looked at the daily figures that he found the expected result- a 1.2 per cent increase in the rate of fatal crashes with every centimetre of rain that falls. Eisenberg says the longer the dry period before it rains, the higher the number of deaths (see Graph), so a dry month with a downpour can have more deaths than one with incessant drizzle. He found that the combination of a shower after a 21-day drought increased the number of fatalities nearly threefold. "I think my study is the first with large-scale data to show the magnitude of the effect," he says. The work will appear in Accident Prevention & Analysis.

Eisenberg suggests that during a drought, engine oil and grease build up so that when it rains the oil and water mix turns the road into a skidpan. And people are also not as prepared for these conditions after a long dry spell, he says. Adrian Runacres, a forensic accident meteorologist at the UK's Transport Research Laboratory at Crowthorne, says that tests in the UK have confirmed that the build-up of dust and rubber from tyres can forma greasy film when it rains. "Also the crud that builds up blocks drainage paths between the chips on the surface of the road," he says. But Runacres says that the most important factor is probably motorists driving in heavy rain as though it were still dry, and tailgating other vehicles. Eisenberg says that highway authorities should warn motorists of the added danger when it rains after a drought.

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Written by Mick Hamer

New Scientist issue: 13th September 2003

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