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Overlap between wildlife and human activity is key to causing wildlife-vehicle collisions, a globally pervasive and growing source of wildlife mortality. 1,2 Policies regarding clock time often involve abrupt seasonal shifts in human activity, potentially influencing rates of human-wildlife conflict. Here, we harness the biannual shift between standard and daylight saving time as a natural experiment to reveal how the timing of human activity influences deer-vehicle collisions. Based on 1,012,465 deer-vehicle collisions and 96 million hourly traffic observations across the United States, we show that collisions are 14 times more frequent 2 hours after sunset than before sunset, highlighting the importance of traffic during dark hours as a key determinant of deer-vehicle collision risk. The switch from daylight saving to standard time in autumn causes peak traffic volumes to shift from before sunset to after sunset, leading to a 16% spike in deer-vehicle collisions. By reducing traffic after dark, our model predicts that year-round daylight saving time would prevent 36,550 deer (Odocoileus sp.) deaths, 33 human deaths, 2,054 human injuries, and US$1.19 billion in collision costs annually. In contrast, permanent standard time is predicted to increase collisions by an even larger magnitude, incurring an additional US$2.39 billion in costs. By targeting the temporal dimension of wildlife-vehicle collisions, strategies such as year-round daylight saving time that reduce traffic during dark hours, especially during the breeding season of abundant ungulates, would yield substantial benefits for wildlife conservation and reduce the social and economic costs of deer-vehicle collisions. Deer-vehicle collisions are 14 times more likely shortly after dark than before Nighttime traffic and deer-vehicle collisions are more frequent during standard time Collisions with deer increase by 16% in the week following the autumn clock change Year-round daylight saving would reduce collisions, saving ∼$1.2 billion annually The shift to standard time each autumn causes an abrupt increase in nighttime driving during the peak breeding season for deer, resulting in a 16% increase in deer-vehicle collisions. Cunningham et al. show that permanent daylight saving time would prevent 36,550 DVCs and $1.2 billion in collision costs annually in the US. [ABSTRACT FROM AUTHOR]