Even the Smallest Amount of Rain Sends Crashes Soaring, Study Finds

It may not be a bombshell report that leaves mouths agape, but it reinforces an age-old bit of driving wisdom: when it starts to rain, slow down and leave a greater distance between you and the car in front.

A new study reveals just how much precipitation plays a role in increasing the likelihood of a fatal crash. Even in weather docile enough to simply dampen one’s hair, death stalks the roadways like a vulture seeking out scraps of rancid meat.

The study, published in the Bulletin of the American Meteorological Society and first reported on by the Associated Press, shows precipitation of all types increases deadly crash risk by 24 percent. In reaching their conclusion, researchers at the North Carolina Institute for Climate Studies probed 125,012 fatal car crashes in the continental U.S. from 2006 to 2011.

This study went beyond the sometimes vague police reports, analyzing the exact precipitation rate at the place and time of the crash using weather radar. While most drivers cut their speed sharply when it starts raining heavily, sometimes just for visibility reasons, the team was surprised to see just how deadly light rain is.

Just driving in light rain — “We’re talking a drizzle, just at the point where you might consider taking an umbrella out,” according to study lead author Scott Stevens — increased the chance of a fatal crash by 27 percent.

The most dangerous time to be driving in any rain, even drizzle, is just after the droplets start to fall. That rain mixes with oil, grease, and other residue on the road surface to make it extra slick, catching motorists off guard. After a sustained rainfall, most of that residue washes away, improving stopping distances. Thus, a road which has just seen a sprinkle can yield worse stopping times than a significantly wetter road that’s been soaked for hours.

Heavy rain, of course, can lead to hydroplaning, endangering even those who aren’t diving for an off-ramp. The visibility issue really comes into play, too. With the road ahead obscured and braking distances lengthened, no longer can drivers depend on the three-second rule.

Moderate rain, Stevens said, boosts the chance of a fatal crash by 73 percent. In heavy rain? It’s two-and-a-half times greater.

Population density also plays a role in the rate of fatal crashes. In heavily congested areas, most drivers aren’t going fast enough to die behind the wheel after something goes awry, even though their vehicle will sustain damage. The study showed that the Northern Rockies and Upper Midwest are the riskiest places to drive when the weather gets bad.

a version of this article first appeared on TTAC