University of Washington: Tweaking Daylight Savings Time Could Stop Many Deer Collissions

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In much of the United States, there is a twice-yearly shift in timekeeping between standard time and daylight saving time, or DST, which delays both sunrise and sunset to make mornings darker and evenings brighter. Recently, scientists, policy experts, lawmakers and citizens have debated abandoning the twice-a-year switch and adopting either year-round standard time or DST.

A team of researchers at the University of Washington — led by postdoctoral researcher Calum Cunningham and Laura Prugh, an associate professor of quantitative wildlife sciences — have found that one of those options would sharply reduce a hazard common to much of the country: deer-vehicle collisions. In a paper published Nov. 2 in Current Biology, they report that adopting permanent DST in the United States would reduce deer-vehicle collisions and likely prevent an estimated 36,550 deer deaths, 33 human deaths, 2,054 human injuries and $1.19 billion in costs each year. Deer-vehicle collisions would decrease under permanent DST because skies would be brighter later into the evening.

There are an estimated 2.1 million deer-vehicle collisions in the United States each year, killing about 440 people, causing 59,000 injuries and costing upwards of $10 billion.

“Wildlife-vehicle collisions are a huge and growing problem,” said Cunningham. “There are social costs — people killed and injured — and it’s also a conservation problem as it’s one of the largest sources of human-caused mortality of wildlife.”

Cunningham, Prugh and their colleagues analyzed wildlife-vehicle collision data from 23 states. The vast majority of these were collisions involving two widespread species of deer, white-tailed deer that predominate in the eastern U.S. and mule deer, which are more common in the western U.S.

The dataset, which includes more than 1 million deer-vehicle collisions from 1994 to 2021, revealed that the risk of deer-vehicle collision depends on the overlap of both human and deer activity. Deer and related species are crepuscular, meaning they are most active around dawn and dusk. The team found that most collisions occurred in the hours between sunset and sunrise the next morning. Collisions were 14 times more frequent two hours after sunset than two hours before sunset.


A deer crossing sign in Folsom, California.California Department of Fish & Wildlife

The researchers found that deer-vehicle collisions peak in the fall, with nearly 10% occurring during the two-week period around the switch from DST to standard time. The shift itself causes an abrupt increase in the amount of driving after sunset, which corresponded with a 16% increase in deer-vehicle collisions in the week following the time shift.

For ungulates like deer, fall is also the “rut,” their mating season, during which their activity level increases by as much as 50%.

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