Across the US, mitigation of the spread of COVID-19 included “shelter-in-place” (SIP) orders and related actions, implemented by cities, counties, and governors’ offices. These orders resulted in a massive drop in traffic volumes and provided an unprecedented opportunity to measure the effect of reduced traffic on crashes, including collisions involving wildlife. For California, the Road Ecology Center at UC Davis processes and compiles real-time traffic incidents from California Highway Patrol reports, using a web-accessible database. Once the governor’s order went into effect, collisions and especially injury crashes were reduced by half, from ~1,000 collisions of all types and ~400 injury crashes per day to 500 and 200 per day, respectively. These rates of crashes increased again as traffic increased, but remained slightly reduced compared to previous years. We found that traffic (as vehicle miles traveled, VMT) declined rapidly by >50% in most US states and stayed depressed for at least one month. Traffic volumes gradually increased again through the summer, but in certain states (e.g., CA) remain depressed by up to 20%, as of April 2021.

High traffic volume is a primary contributor to wildlife-vehicle conflict (WVC) and wildlife mortality on roads. We investigated how reduced traffic affected WVC. Using traffic and collision data from four US states (California, Idaho, Maine, and Washington), we investigated changes in total WVC, following the state and local SIP orders. The daily WVC rates declined 34% between the 4 weeks prior to SIP orders going into effect, to the 4 weeks after, with 21, 36, 44, and 33% declines for CA, ID, ME, and WA, respectively. The reductions in WVC from 1 month pre-SIP orders to 1 month post-order only occurred in 2020 and not 2015, 2016, 2017, 2018, or 2019, suggesting that the reductions were associated with the reductions in traffic. Mountain lions (Puma concolor) are proposed for listing as endangered in CA and for them there was a 58% decline in mortality during the traffic reduction. The measured declines in WVC reversed in ME and WA during May, June and July, 2020, paralleling reversals in traffic volumes. A 34% reduction in WVC would potentially equate to 10s of millions fewer vertebrates killed on US roadways during one month of traffic reduction, representing an unintentional conservation action unprecedented in modern times. There is no equivalent in our recent transportation history to such large changes in vehicle movement on our state and local roads. The direct relationship between traffic and rates of all crashes and WVC in particular highlights the importance of mitigating continuing traffic impacts on human and wildlife safety.