Overall, we found that while traffic volume decreased > 7% during the pandemic year (with a maximum monthly decline of almost 40%), the absolute number of annual WVCs remained broadly unchanged. This resulted in a significant increase of >8% in the number of collisions between vehicles and wildlife during the pandemic year, peaking at >27% nationwide in April 2020. Other studies from the first few months of the pandemic documented similar transient declines in the number of WVCs as the pandemic began, which then reversed in many jurisdictions as the pandemic progressed and traffic recovered26†27† We observed a similar pattern during the first five months of the pandemic on a national scale (Fig. 2): WVCs initially declined during the pandemic along with the decline in traffic volume, but then began to rise to baseline levels at a faster rate than traffic, possibly due to behavioral slowdowns in wildlife following traffic-mediated increases in the use of the road for wildlife. Although based on coarse data, our study is consistent with claims made during27 and prior to the pandemic3†15†16†28†29 that the relationship between traffic volume and WVCs is non-linear.
We postulate that the observed nonlinear relationship between traffic volume and WVCs is the result of greater use of roads and verges by certain animal species, namely large mammals (Table S1), in response to declining traffic volume, as previous research has suggested3†14†15†16† This explanation is consistent with stories of different animal species making greater use of human spaces during the pandemic17†20†21: With fewer cars on the roads, wildlife may be less likely to be deterred from the road by the noise and light pollution associated with high traffic volumes9†10†11†20 and roads as less risky, increasing their willingness to attempt crossings3†8†15†16† Beyond the occasional road crossing as you move through the landscape8†9wildlife can be attracted to roads to travel, friends or other resources8†10†11† Many animals have been shown to use roads to move efficiently through the landscape11†12and roads and surrounding areas are relatively open, allowing wildlife to select roads and berms for better visibility to find mates, detect predators, or locate prey10†13† Roadsides can also provide foraging opportunities and vital nutrients for wildlife through abundant, high-quality early successive vegetation and high salt concentrations10†11† As such, reduced road traffic during the pandemic may have caused certain animal species to tolerate the risks of roads in order to access the benefits of roads and berms.
An alternative explanation for the observed increase in the number of collisions is that human driving behavior has changed rather than animal behavior during the pandemic. With fewer cars on the road, people may be driving faster35making it more difficult for both humans and animals to avoid collisions3† Indeed, preliminary studies from across the United States have suggested changes in people’s driving behavior during the pandemic, with several jurisdictions reporting higher vehicle speeds.35†36† Despite reported increases in vehicle speeds, the total number of vehicular collisions (the sum of both wildlife and other collisions) reflected traffic volume trends and decreased significantly during the pandemic37†38† Thus, because changes in human behavior appear to have minimal effect on vehicle collisions in general, it is unlikely that the observed changes in collision rates are due to increased vehicle speeds alone. Still, we cannot rule out the possibility that changes in human driving behavior contributed to the patterns documented here, and future work should test more explicitly the relative effects of changes in traffic volume on both human driving behavior and wildlife space use. as well as the resulting effects on WVCs.
A better understanding of human driving behavior would also help explain our findings regarding changes in traffic patterns during the pandemic. Nationally, the severity of the COVID-19 restrictions accounted for much of the variation in changes in monthly traffic volume (R2= 0.968), but disability severity had less of an impact on changes in annual interstate traffic (Tables S3 and S4† Restrictions introduced during the pandemic were largely introduced with the aim of minimizing travel, and other research has shown that these restrictions were effective in reducing human mobility.18†21† However, our state-level findings imply that it was not only the limitations themselves that reduced travel, but possibly the associated fear related to the risk of contracting the SARS-CoV-2 virus, as suggested in other studies.21†22†23†24† although we observed the greatest decrease in traffic volume early in the pandemic (Fig. 2A) when the restrictions were strictest (Fig. S2†21there was widespread concern about the risks of SARS-CoV-2 . during this period22†23that probably motivated people to stay at home regardless of disabilities24† Indeed, anxiety and risk perception could explain the relationship between traffic volume and the other covariates in our top models (Table .). S4† The drop in traffic was greatest in the most populous states (Fig. 4A) and in states with the highest and lowest burden of disease (Fig. 4b). The risk of SARS-CoV-2 transmission is greater in more densely populated states due to proximity and frequent interactions between people21† As such, it is possible that people have changed their road use more in densely populated states than in sparsely populated states due to different perceptions of disease transmission risk.23-although differences in infrastructure in relation to population density probably also contributed to this pattern39† Similarly, declines in traffic volume in states with larger SARS-CoV-2 outbreaks may have been caused by an increase in the perceived risk of contracting the virus.21†23† Alternatively, traffic reductions in states with a low burden of disease may reflect greater compliance with home orders, and thus less chance of disease spreading40†41† essentially, reduction in traffic volume may be the cause of locally low disease burdens therein, rather than a consequence. Overall, we argue that the observed heterogeneity in traffic volume between states is attributed, at least in part, to differences in the perceived risk of SARS-CoV-2 virus.
Regardless of the mechanisms underlying changes in traffic volume and WVCs, our observation that the annual number of WVCs was largely unchanged despite significant decreases in traffic volume has implications for reducing WVCs in the future. Most directly, the lack of a change of direction in WVCs indicates that road traffic in the United States is currently such that even a large decrease in traffic volume would have minimal long-term effects on the absolute number of WVCs. As such, reducing collisions by reducing traffic volume would require even greater and longer-lasting changes in traffic than during the pandemic. Since such a massive and sustained reduction in traffic is unlikely4†5†6As of now, WVCs in the United States represent essentially a fixed cost, both to human society and to wildlife populations. As such, these transient decreases in traffic likely minimized reprieve for large mammals from collision-induced mortality, contrary to speculation that changes in human mobility during the COVID-19 pandemic had significant positive effects on wildlife populations by freed wildlife from the ubiquitous direct and indirect effects of humans17†18†19†20†26†27†42†
Indeed, it is possible that in the short term a reduction in traffic volume could ultimately be harmful to the animal species that have increased their road use. While the increases in the number of collisions we observed at the start of the pandemic were rapid and consistent with a nationwide decline in traffic volume (see also26†27), collision rates remained high even as traffic approached baseline levels in July (Fig. 2b). If wildlife responses to traffic changes are asymmetrical (e.g.,† increase in the use of wildlife on the road due to a decrease in traffic occurs faster than a decrease in wildlife use in response to the increase in traffic), then a decrease in traffic volume in the short term could lead to a net increase in the number of WVCs over a longer period of time, which can ultimately be harmful to certain wildlife populations1†3† Future work should evaluate the long-term effects of the pandemic on wildlife populations, particularly with regard to collision-induced mortality17†20†26†27†42†
While the COVID-19 pandemic provided an opportunity to explore the short-term effects of transient decreases in traffic volume on WVCs, the long-term effects of growing human populations, increased road densities and overall higher traffic volumes on WVCs are less clear. Like the increase in road use for wildlife in response to the decrease in traffic volume theorized here, a steady increase in traffic may reduce road use for wildlife in the long run.3†14†15†16† as road traffic does indeed increase with time4†5†6can we therefore see declines in WVCs as roads become more effective at repelling wildlife1†3†14† While these reductions in vehicular wildlife deaths are welcome, they would result in roads becoming more and more a barrier to animal movement and gene flow.43further fragmentation of already disconnected wildlife populations8† Policymakers and urban planners should therefore invest in infrastructure such as flyovers, underpasses and fencing that allow wildlife to safely cross busy roads or direct wildlife to low-risk areas.8†9† Even a significant short-term decline in road traffic is not enough on its own to reduce conflict between wildlife and vehicles.