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Warmer temperatures, not cell phone use, was behind recent spike in road deaths, researcher says

MinnPost photo by Corey Anderson
When weather is warmer, people tend to be out on the streets more — not just in cars and other motor vehicles, but also on bicycles and on foot.

Hotter than normal temperatures — not cell phone use — may have caused the unusual spike in road deaths that occurred in the United States in 2015, according to an intriguing study published last week in the journal Injury Prevention.

Road deaths in the U.S. jumped 7 percent in 2015, to 35,200 — an abrupt reversal of the downward trend that has persisted for the previous 35 years. Some safety experts have attributed that increase to drivers’ use of cell phones. But Dr. Leon Robertson, a retired Yale University injury epidemiologist and prevention expert and the author of the new study, wasn’t convinced by that explanation.

“While cell phone use is associated with an increased risk of severe crashes, cell phone ownership increased gradually and approached saturation among the US population several years before 2015,” he writes. “A national probability sample of drivers observed in traffic during 2014 and 2015 found no increase in use of all hand-held devices and a slight non-significant decrease in hand-held cell phone use from year to year.” 

“Use of such devices while driving likely dampened the rate of reduction in death rates in recent decades but it is unlikely to account for the abrupt change in road deaths in 2014-2015,” he adds.

Study details

Robertson instead believes that the reversal in the downward road-death trend might be the result of climate change — specifically, the unusually large average annual increase in temperature (1.5 degrees Fahrenheit) increase that occurred in 2015. 

For when weather is warmer, people tend to be out on the streets more — not just in cars and other motor vehicles, but also on bicycles and on foot.

To determine the merits of his hypothesis, Robertson examined two large sets of data. One allowed him to assess the association between temperature and rainfall on the annual miles driven in the U.S. per person in urban areas. The other helped him link weather and other factors to the number of road deaths per person in the 100 most densely populated counties in the U.S.

He found that for each degree increase in temperature, vehicles were driven an average of 60 extra miles per person per year. He also found that for each additional inch of rainfall, vehicles were driven an extra 66 miles per person.

That added up to 13.6 billion extra miles driven in 2015 in the urban areas covered in the study. “And that doesn’t include the likelihood of increased bicycling and walking in warmer periods,” Robertson adds.

Further analysis revealed that the rate of road deaths was higher in warmer counties and in those with higher rainfall. Robertson then calculated that more than half of the variation in death rates between the years 2014 and 2015 could be explained by temperature — even after adjusting for such factors as precipitation, speed limits and driving-while-under-the-influence (DUI) laws. 

Counties with higher average household incomes tended to have lower death rates, Robertson also found — perhaps, he says, because they are able to afford newer models of cars with more safety features.

Limitations and implications

This is an observational study, so it can’t prove cause and effect. Also, as other experts told MedPage Today reporter Alexandria Bachert, Robertson’s analysis included some assumptions that need further exploration.

“What about seasons, day length and sun levels?” asked one of those experts. “There needs to be more research in the area.”

Robertson believes, however, that “as temperatures continue to increase from heat-trapping gases in the atmosphere, road deaths will likely increase more than expected unless there are major mitigating countermeasures.”

What are those countermeasures? Lowering maximum speed limits on urban freeways and installing speed bumps in residential areas would be good places to start, he told MedPage Today.

Greater separation of motor vehicles from cyclists and walkers is also needed, he added.

“Walking and bicycling are healthful exercises but are best pursued on off-road trials, parks, athletic fields, malls, etc.,” Robertson explained. “Some cities provide biking and walking venues separate from traffic. Where such are not feasible, barriers between motor vehicles and pedestrians or bicyclists are an option.” 

For more information: You’ll find an abstract of the study on Injury Prevention’s website, but the full study is behind a paywall.

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