More rain, snow makes driving safer?

By: Drivers.com staff

Date: Tuesday, 29. June 2010

Do the first snows of really winter bring a rash of crashes? Is rain after a long dry spell more dangerous? Our intuitions tell us that the answers to these questions are "yes" and "Yes," but recent research from the University of California at Berkeley adds some very interesting insights into the relationship between these adverse conditions and statistics for crashes.

In a study of more than one million crashes in 48 U.S. states, Researcher Daniel Eisenberg was surprised to find that the more it rained or snowed in a month, the fewer deadly traffic accidents there were. And he found that in any given month, an additional 10 centimeters of rain is linked with a 3.7 percent decrease in the fatal crash rate!

Eisenberg's findings [PDF] might seem counter to popular belief about the risk of crashes but there's good solid common sense behind them - it's not so much the rain or snow that causes crashes but drivers' abilities to adapt to change. "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," Eisenberg said.

When he looked at the amount of rain in a given month, Eisenberg expected to find that the more it rained the more the likelihood of fatal crashes. What he did find was the opposite. Specifically, in any given month, an additional 10 centimeters of rain was associated with a 3.7 percent decrease in the fatal crash rate.

He also discovered that the longer the dry spell preceding the onset of rain the greater the risk of crashes when the rain came.

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

The first day of rain, snow

It wasn't just monthly data on snow and rain that showed the negative correlations. The same associations applied to annual statistics as well. However, when Eisenberg looked at daily accident data, he got surprising results, although those results might not have surprised the driver ed teacher who told you years ago about the risks of dry spells and how accumulated dust and oil can create a deadly slick surface.

Eisenberg's research showed that more precipitation on a given day was linked to a higher number of fatal traffic accidents, but when he looked more closely at his data he found that "the effect of precipitation on accidents on any given day depends upon how much it rained or snowed prior to that day."

"For instance, if it rains a centimeter today, on average, there will be no increased risk of fatal crashes if it also rained yesterday," Eisenberg said. "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 says. "What my study does is provide hard evidence to support it and to quantify just how much the weather changes the risk."

Eisenberg's study also included non-fatal crashes and his results were much the same. Analyzing 36.4 million nonfatal crashes from 1990-1999 in the 17 states for which records were available. He found that precipitation had a larger impact on nonfatal traffic accidents.

"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.

Eisenberg's results will be published in an upcoming issue of the journal Accident Analysis & Prevention.

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