THE WORST MONTH FOR U.S. TRAFFIC DEATHS

Most people would assume that the summer months are the months with the most traffic accidents. Summer travel, July 4th festivities, barbecues, and more liberal use of alcohol are all prevalent. However, that intuition would be wrong.

It is September.

According to separate studies, there are more traffic deaths per vehicle mile travelled in that month than in any other of the year.

From 2009 through 2018, there were, on average, 12.3 traffic fatalities per billion miles driven in September, according to Michael Sivak, a transportation analyst with Sivak Applied Research. October was the second worst month with a dozen deaths per billion miles driven. March was the least risky with a death rate of 10.3—or 19% fewer than in late summer and early fall. The Insurance Institute for Highway Safety, a nonprofit funded by auto insurance companies and dedicated to reducing losses from motor-vehicle crashes, documented the same pattern from 1998 through 2014, finding that an average of 14.2 people died per billion vehicle miles driven in September.

Although the overall number of U.S. traffic deaths has fallen in recent years, more than 35,000 people still die annually in crashes, according to figures provided by the Insurance Institute. In general, most traffic fatalities occur in the summer and fall and around holidays when more people are on the road. Weekends are worse than weekdays and more crashes tend to happen between 3 p.m. and 9 p.m.

An average of 139 lives were lost each Saturday in the period studied by the Insurance Institute, the most of any day of the week. The fewest deaths, an average of 89, occurred on Tuesdays.

With the exception of New Year’s Day, when an average of 135 people died, the worst dates to drive were in the summer, with an average of 141 deaths on July 4, 130 on July 3 and 125 each on Aug. 2 and Aug. 3.

“Basically, late summer and early fall is the most risky time of year, and late winter and early spring is just the opposite,” Dr. Sivak said. “There is some evidence people drive slower in winter than they do in the summer. They are compensating for the inclement weather, and if they drive slower and then crash, they’re less likely to be killed.” Based on raw numbers, August and July were the worst months for traffic fatalities with an average of 116 people dying each day. The daily average for September was 112, but when the researchers accounted for the number of vehicle miles traveled in each month, September rose to the top. The researchers aren’t sure why, but they mentioned a few factors that might contribute. “In general, we tend to see more deaths in nicer weather,” Dr. Monfort said. “People are driving faster and more recklessly. They go out more.” September is the tail end of summer, when people are still going out, he said, and it’s also when it begins to get dark earlier, making driving more challenging for young drivers with less experience behind the wheel and older drivers who have difficulty seeing at night. The long Labor Day weekend might also play a role.

This year, fatalities could be worse. “Not only are we getting into September, which is associated with higher-risk travel, but we’re also rebounding from COVID-19 lockdown measures,” he said. “We might not see the drop in vehicle miles traveled that normally happens around this time. It’s something people should be aware of and maybe drive more carefully.”