Berkeley - Going out for a drive in the rain? A study by a researcher at the University of California, Berkeley, provides new data on how risky that road trip might be.

In an analysis of more than 1 million fatal crashes in 48 states, Daniel Eisenberg, a post-doctoral researcher at UC Berkeley's School of Public Health, was surprised to find that the more it rained or snowed in a month, the fewer deadly traffic accidents there were. Specifically, in any given month, an additional 10 centimeters of rain is linked with a 3.7 percent decrease in the fatal crash rate.

"I had expected to see a positive relationship between the amount of precipitation and the rate of fatal traffic accidents, but my analysis revealed a more complex connection between the two," said Eisenberg.

He also discovered that the risk of an accident on a rainy day increases with the length of the dry spell preceding it. If there has been rain or snow day after day, the danger due to wet conditions falls.

The results will be published in an upcoming issue of the journal Accident Analysis & Prevention, but are now available online to subscribers.

Eisenberg obtained weather data from the National Climactic Data Center, and traffic crash records from the National Highway Traffic Safety Administration (NHTSA) for the study. He analyzed fatal crashes from 1975-2000.

Not only monthly, but annual data showed Eisenberg that more rain and snow was linked to fewer fatal crashes. But when he looked at daily accident data, he found that more precipitation was linked to a higher number of deadly traffic accidents.

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Further analysis showed that the "the effect of precipitation on accidents on any given day depends upon how much it rained or snowed prior to that day," he said.

"For instance, if it rains a centimeter today, on average, there will be no increased risk of fatal crashes if it also rained yesterday," said Eisenberg. "But if it's been two days since the last rain, then the risk for a deadly accident increases by 3.7 percent. If you take it out even further to 21 days, the risk increases to 9.2 percent, or about two and a half times more risk than if it only rained two days before."

Eisenberg pointed to two likely explanations for this result. "Oil and debris accumulate on the road when it hasn't rained for a while, making the roads slicker when it first starts to rain," he said. "By the second day of rain, the oil and debris have washed off the roads and are less dangerous. Another factor could be that people aren't as used to driving in the rain when it comes after a long dry spell. Perhaps they become better adapted to the weather conditions by day two or three."

"Many people are already aware of this phenomenon," Eisenberg added. "What my study does is provide hard evidence to support it and to quantify just how much the weather changes the risk."

In addition to analyzing deadly accidents, Eisenberg looked at 36.4 million nonfatal crashes from 1990-1999 in the 17 states for which records were available.

He found that, overall, precipitation had a larger impact on nonfatal traffic accidents.

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"For any given day in the state, on average, each centimeter of precipitation increases the risk of fatal crashes by about 1 percent, but for nonfatal crashes, the increased risk is 11 percent," said Eisenberg.

So, on any given day, rain or snow will lead to increases in nonfatal injury crashes and fender benders much more so than to increases in accidents that involve death.

"People who slow down when the weather is bad may not slow down enough to avoid all crashes, but, on average, they at least reduce the severity of the collision," said Eisenberg.

The results of the study suggest that transportation departments should consider using electronic roadside warning signs that emphasize the risk during the first rain or snow following a dry spell, said Eisenberg. He also said that lower speed limits in those situations may be beneficial.

"I wouldn't be surprised if these kinds of measures proved to be effective in improving safety, and at a relatively low cost," said Eisenberg.

The National Institute of Mental Health provided funding for this research.