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Fascinating Graph Shows How Crashes Increase When It Rains In L.A.

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Graph showing the average car crashes per hour in L.A. The blue bars represent crashes that take place during rain, and the red bar when it is dry (Image courtesy of Noah Deneau)
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Although it's not earth-shattering news that drivers get into more car crashes when it's raining than when it's not, it is fascinating to see just how much more. An Austin-based electrical engineer put together a nifty and easy-to-read graph that shows how much wreckage Angelenos get into on the roads during rainy days.

Noah Deneau gathered 11 years-worth of data from the NOAA weather reports and California's Statewide Integrated Traffic Records System (SWITRS) to calculate the mean crash rate per hour when it's rainy and also when it's dry. In the 24-hour bar graph above, the red bars represent the crashes during dry periods and the blue stands for the rainy periods with wet roads. The graph is divided between five, ten, and more than ten crashes per hour. The number of crashes in both dry and wet weather increase during rush-hour traffic.

Los Angeles is the first city that Deneau has chosen as his subject in this project. He tells LAist how he first came up with the idea: "I was wondering if the wet/dry traffic accident rate differs much by city, especially places with infrequent precipitation like L.A. versus rainy locales like Seattle. (We in Austin are also no stranger to long dry spells followed by super-slick roads at first rainfall.) I saw a post referencing California's SWITRS crash reporting system, and with L.A. having plenty of traffic—and crashes—I thought it would be an interesting place to start."

He says that he hasn't looked at other cities yet, so this graph doesn't say much about how Angelenos compare to other folks in rainy car crashes. And that means we can't say for certain that L.A. people just don't know how to drive in the rain. Jalopnik put together some theories on why they think Angelenos get into so many car crashes including the fact that it rains so infrequently that grease and debris builds up on the roads and when it rains, it all becomes a slippery danger zone. "Another is that drainage optimized for desert conditions simply can't cope with the volume of a temporary deluge," according to Jalopnik.

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However, NOAA contacted Deneau, telling him that they're working on a similar study on how weather correlates with deaths from car crashes, so there may be some new data to work with in the future, Deneau says.

As for his methodology, what Deneau counted as a rainy hour was one that had "measurable rainfall" that was more than just trace amounts, and also counted the hour after the rainfall as a rainy period to take into account the time the roads would need to dry, he wrote on Reddit. He did note that he didn't account for the distance from the crash to the weather station where the weather was recorded. So it's possible it would have rained at the weather station, but not where the crash happened.

[h/t: CityLab]