Fall is often associated with vibrant colors and pumpkin pie. Now you can add car-crash fatalities to the list, according to a study by the University of Michigan.
Michael Sivak, research professor and head of the Human Factors Division at the University of Michigan Transportation Research Institute, found that the deadliest months to be on the road are September through December. Analyzing data from the U.S. Department of Transportation, he discovered that fall months have the highest car-crash death rates.
 |
Month |
Estimated fatalities per billion kilometers |
January |
9.45 deaths |
February |
8.96 deaths |
March |
8.78 deaths |
April |
9.1 deaths |
May |
9.41 deaths |
June |
9.64 deaths |
July |
9.94 deaths |
August |
10.07 deaths |
September |
10.15 deaths |
October |
10.2 deaths |
November |
10.18 deaths |
December |
10.15 deaths |
Source: Michael Sivak, University of Michigan Transportation Research Institute, "Traffic Injury Prevention, 2009." The monthly fatality rates were analyzed for a 13-year period from 1994 to 2006
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"I was not surprised by the findings. However, most people were," says Sivak. "The general expectation is that winter is the riskiest time of year [due to snow and ice]."
Sivak calculated the fatality rates per distance driven for each month from 1994 to 2006. October ranked at the top, with 10.2 deaths per billion kilometers (roughly 621 million miles). March snagged the lowest rate at slightly less than 8.8 deaths per billion kilometers, followed by February and April.
"The risk of a fatality per distance driven in October is about 16 percent greater than the risk in March," says Sivak. "Everything else being equal, inclement weather — snow and ice — should increase the risk of [crashing]. However, because inclement weather also leads to general reductions in speed, the net effect is not clear."
One possibility for fall's high car-crash death rates could be the lack of daylight, he says. The fatality rate in darkness is substantially greater than the rate in daylight. The duration of darkness increases in the fall, but Sivak adds that pinpointing a single reason is difficult.
"There are several known factors with major influences on the risk of driving that show strong seasonal variations," he says. "However, the peaks and troughs of the seasonal variations of these factors do not fully match the pattern of the overall driving risk. Thus, the driving-risk pattern is likely a consequence of joint contributions of several factors."
Besides weather and lack of daylight, other factors that have major influences on crash risk include consumption of alcohol, the proportion of elderly drivers on the road and the amount of leisure driving people do, according to the study.
It would then be reasonable to think that summer months would be deadlier than fall because that's when most people drive for leisure, Sivak says. Compared to commuter driving, leisure driving is riskier because it's more often done on unfamiliar roads, possibly at higher speeds and during night hours, and at times under the influence of alcohol. Yet statistics still point to the fall as being the deadliest.
The study also states that driving is most risky in the late evening and early morning, especially on Friday and Saturday nights. But the good news is that car-crash fatality rates throughout the period examined have decreased. In 1994, there were 10.7 deaths per billion kilometers driven. That number dropped to 8.8 deaths per billion kilometers in 2006.
