AP Photo/Rich Pedroncelli

Every 10 percent rise in fuel costs leads to a 1.5 percent decline in crashes nine months later, according to new research.

America is home to some of the cheapest gas in the world because U.S. fuel prices fail to capture the full social cost of driving—externalities like congestion, air pollution, and traffic deaths, to name a few. In (yet another) sharp post on the topic, Joe Cortright at City Observatory points us to a very instructive study that sheds new light on how safety suffers when pump prices plunge. Or, if you prefer rosier goggles, how higher gas prices lead to fewer crashes.

There are many factors related to road safety, from infrastructure design to highway maintenance to and distracted or drunk driving, but economic conditions play a huge role. During hard times, or when gas prices surge, people drive less: some shift to cheaper travel modes, some just stay home. These effects tend to “lag,” in econ lingo, meaning they don’t kick in right away; during a recent gas spike in Atlanta, for instance, bus ridership jumped 32 percent six months later.

One predictable and well-documented result of big spikes in gas prices is fewer car crashes, but the precise lag time of these effects has been a bit of an open question. For the recent study, published in the American Journal of Public Health, researchers gathered Mississippi crash data from April 2004 to December 2012 on a month-by-month basis. Then they collected gas prices over this period as well to see when safety connections emerged.

The short answer: about nine months after the fact. Here’s the upshot, via AJPH:

In other words, if gasoline prices increased in a particular month, there was
a consequent decrease in traffic crashes 8 to 10 months later.

For every 10 percent increase in gas prices, the lagged effect produced a 1.5 percent decrease in traffic crashes per capita. That relationship held even controlling for other factors that influence driving trends (such as unemployment, seat belt use, alcohol consumption) and adjusting for seasonal driving patterns as well. It was generally consistent across gender, race, and age, with a notable exception being an immediate effect (meaning no lag) for teenage drivers.

The researchers extrapolate the findings to estimate what the broader safety impacts would be at either end of the gas price spectrum. If fuel costs had been at their lowest point ($1.81) over the entire study period, the researchers would expect 57,461 more crashes to have occurred—a 5.7 percent rise. But if fuel costs had been at their highest ($4.17) during this time, the expectation would be for 70,655 fewer crashes, or a 7 percent decline.

(Chi et al, AJPH, 2015)

The findings are yet the latest reminder that the most direct way to influence driving is to change its price via elevated gas taxes. It’s not a simple policy discussion; higher fuel costs do put a serious squeeze on working-class households. But public officials who claim to care about your health on roads or your time in traffic are being disingenuous unless they give real consideration to raising the costs of driving that directly bear on these outcomes.

Until that happens—and don’t hold your breath—it’s worth remembering every time prices go up at the pump that there’s a silver lining just a few months down the road.

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