by Michael Sivak.

Ten years ago, I published an analysis showing that the risk of driving in the United States was lowest during late winter and early spring and highest during fall. That analysis covered years 1994 through 2006. The present study examined whether this pattern still holds.

The calculations used the monthly road fatalities from the National Highway Traffic Safety Administration and the monthly distances driven by all vehicles from the Federal Highway Administration. The examined data were for the 10-year period from 2009 through 2018. The chart below shows the results.

The results indicate that the monthly pattern of fatalities per distance driven for 2009 through 2018 is similar to that for 1994 through 2006, with the rate being lowest during late winter and early spring and highest during fall. The minimum in the present study was in March (10.3 fatalities per billion miles) and the maximum in September (12.3 fatalities per billion miles). The September rate was 19% higher than the March rate. (The two extremes in the earlier study were in March and October.)

The seasonal variation in the road fatality rate is likely a result of complex joint influences of several factors that exhibit different seasonal variations themselves (such as the proportion of rural to urban driving, consumption of alcohol, age distribution of drivers on the road, proportional amount of leisure driving, duration of darkness, and inclement weather).

Michael Sivak is the managing director of Sivak Applied Research and the former director of Sustainable Worldwide Transportation at the University of Michigan.