Permanent daylight saving time could prevent deer-vehicle collisions, scientists say

The autumnal switch from daylight saving time (DST) to standard time comes with a surge of deer-vehicle collisions in the U.S., a new study suggests. The researchers argue that sticking with DST, permanently, could reduce the number of accidents, sparing the lives of dozens of people and tens of thousands of deer annually.

The new study, published Wednesday (Nov. 2) in the journal Current Biology, examined how switching to DST in spring and then turning the clocks back in fall might affect the rate of animal-related car crashes. Data from 23 states suggested that more than 90% of these crashes involve deer, most commonly either white-tailed deer (Odocoileus virginianus) or mule deer (Odocoileus hemionus), according to a statement.  

Deer tend to be most active around dawn and dusk, but according to the collision data, the risk of crashes jumps most significantly just after nightfall. Collisions occurred 2.3 times more frequently in the two hours after sunset than in the two hours before sunrise, and crashes were 14 times more frequent after sunset compared with two hours before sunset. 

Overall, the rate of deer-vehicle collisions peaks in autumn, "spiking in late October through November in all states analyzed except Alaska," the team wrote in their report. Roughly 10% of all the reported deer-vehicle collisions occurred in a two-week window after the "fall back" to standard time. Traffic volume data collected between 2013 and 2019 suggested that the time change comes with a sudden increase in the amount of driving after sunset, when deer-vehicle collisions are most likely.

Related: Why it's time to abolish daylight saving time 

This sharp increase in crashes mostly affects the East Coast, senior author Laura Prugh, an associate professor of quantitative wildlife sciences, told Science News. "In the western states, you also see an increase, but it's not nearly as sharp," she said. This is likely because the mating season, or "rut," for white-tailed deer peaks around the fall time change and the animals are more common on the East Coast than on the West Coast. Mule deer, which are more common in Western states, typically reach peak mating season later in the fall and early winter.

"We believe that this fall spike really happens due to the overlap of these two factors: the breeding season and the change from daylight saving time back to standard time," Prugh said in the statement. "We don't see a corresponding shift in deer-vehicle collisions in the spring during the other time change, and we believe that's in part because spring is not a breeding season for deer."

Prugh and her colleagues calculated that, if the U.S. switched to permanent DST, it could prevent about 2.3% of the 2.1 million deer-vehicle collisions that occur each year. That would prevent an estimated 33 human deaths, 2,054 human injuries, 36,550 deer deaths and $1.19 billion in costs annually, according to the statement. 

That said, the decrease in collisions would not be evenly distributed across all states, the authors wrote. The estimated change in the rate of crashes ranged from "an increase of 2.5% in Kansas to a decrease of 8.3% in Maine."

However, the reverse scenario — switching to permanent standard time — was projected to increase the number of collisions in every state, the team reported. In total, the team estimated that the switch would cause 73,660 more deer-vehicle collisions each year, leading to 66 additional human deaths, 4,140 more human injuries, 74,000 more deer deaths and an added $2.39 billion in costs annually.