John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
Crunching the numbers on L.A.'s wet-weather car wrecks.
The inability of California drivers to handle the rain is legendary: From San Francisco to L.A., there's no better way to majorly screw up a commute than by throwing in some drizzle.
What has long been a regional stereotype now has numerical support from Noah Deneau, an electrical engineer and visualization enthusiast in Austin. Deneau has taken 11 years of NOAA weather reports and accident data from California's Statewide Integrated Traffic Records System and crafted a simple, yet enlightening look at L.A.'s rain-derived auto carnage. The red bars here indicate the mean crash rate per hour when conditions are dry; blue bars represent rates when things are wet:
Fair-weather crash rates peak at 10 an hour for three hours in the early rush, but stay much lower through the rest of the day, according to Deneau's accounting. Meanwhile, accidents during rainy periods soar above 10 for more than half the day, approaching 15 per hour around 3 p.m. As for why this happens, take your pick from the following theories on Reddit: Californians tailgate too much; they're so used to drought they've forgotten how to drive in rain; the state's arid weather causes copious build-ups of roadway oil, creating slippery, hazardous surfaces when the drops start falling.
While the rain-wreck correlation seems clear, there are a couple of limitations to this analysis. One, the rain reports are sourced from local weather stations and not accident sites, meaning that some "wet" crashes might've occurred in dry weather. "L.A. covers a big area split by mountains," says Deneau, "and it's highly possible it's raining at the station in downtown but sunny in Van Nuys, for example."
It also doesn't take into account how traffic might swell during showers and storms. "I was mainly trying to show how there are more wrecks during rain," Deneau says. "If you assume for any given hour of the day, roughly the same number of cars are on the road no matter if it's raining or dry, this chart is valid. (Whether that's a safe assumption to make I don't know.)"
However, it's a line of inquiry intriguing enough to attract the government's own data-crunchers, says Deneau. "A fellow from NOAA actually contacted me and said they're working on a similar analysis nationwide looking at traffic fatalities and weather," he says, "so hopefully we'll see some nice data from them in the near future."