Species characteristics affecting WVCs include the nearby population density, the crossing willingness or entering opportunity, the crossing ability, and morphological and life-history traits. Morphological and life-history traits include mobility, body size, diet type, gender, age, health, body length, group size, various road uses (e.g., foraging or thermoregulation on roads), reproductive and breeding behaviors, seasonal migrations, post-breeding dispersal, and movements to hibernation locations (Table 1).

^][100]. There is also no consistent conclusion about the relationship between diet type and the risk of WVCs for mammals, as the dominant factors considered in different studies ^{[8][9][10][13][14][15]}[80,92,95,96,98,99], such as mobility, population density, and foraging on roads, were not identical. The gender difference in the WVCs of certain species mainly results from the different crossing frequencies between males and females (bias toward males: e.g., Refs. ^[16][17][18][29,31,33]; bias toward females: e.g., Refs. ^[19][34]). Crossing roads in groups may reduce the risk of WVCs because of increased vigilance ^[14][96]. Nevertheless, Pfeiffer et al. (2020) pointed out that when white-tailed deer (Odocoileus virginianus) cross roads in groups, despite the increased total vigilance of the group, the vigilance of individuals in the group might decrease ^[20][101]. In addition, individuals in large groups tend to catch up with others on opposite sides of roads, thus increasing the risk of WVCs ^[21][93]. As a rare study involving the relationship between health conditions and WVCs, Møller et al. (2011) found that the risk of WVCs is higher in bird species with blood parasite infections ^[11][84]. Various road uses, such as foraging (e.g., predation, scavenging), movement, and thermoregulation, can increase the density and road crossing of wildlife, which may enhance the risk of WVCs ^[22][23][2,5]. For example, the presence of prey on roadsides may increase the risk of WVCs of predators ^[24][25][94,97]. In northern latitudes, just after dawn, birds often sit on tarmac roads, which are warmer than dew-laden roadside vegetation.

Road and traffic characteristics affecting WVCs include road design, road age, traffic volume, and vehicle speed. Road design includes the road class, road width, road curvature, road embankments, road verges, medians, road lights, roadside vegetation, and mitigation measures (Table 2).

Numerous studies have found that the local population density is a crucial factor influencing WVCs ^{[1][2][3][4][5][6]}[86,87,88,89,90,91]. As mentioned previously, larger species and carnivores are likely to be more mobile, but the specific effects on WVCs must be considered in combination with other factors. Several studies indicate that medium-sized mammals have the highest risk of WVCs, and the comparatively lower risk of WVCs of larger mammals can be explained by a lower population density, better crossing ability, and higher visibility for drivers ^{[7][8][9][10]}[85,92,95,98]. Regarding birds, Møller et al. (2011) and Medrano-Vizcaíno et al. (2022) found that the WVCs of birds are positively correlated with body mass ^[7][11][84,85], while Morelli et al. (2020) came to the opposite conclusion ^[12

Wider roads mean spending more time crossing, a higher traffic volume means fewer intervals to cross, and a higher vehicle speed means less time to react ^[26][27][28][62,115,116]. Although curves may lower the visibility of drivers ^[29][103], they may also reduce vehicle speed ^[30][31][60,128] and enhance drivers’ attention ^[32][121]. Indeed, the majority of studies indicate that WVCs are positively correlated with road width, number of lanes, road class, traffic volume, and speed limit and are negatively correlated with road curvature ^[33][21]. Conversely, a high traffic volume can reduce the crossing willingness of some species and lower the risk of WVCs ^{[3][26][30][34][35][36][37][38]}[58,60,62,88,134,135,136,137]. Additionally, it should not be overlooked that high traffic may depress the nearby population density via high road mortality, which causes road-kill hotspots to move to low-traffic segments ^[39][40][55,138]. Furthermore, high traffic may also make corpses disappear faster ^[39][55]. The negative relationship between the road width and the WVCs of certain species can be primarily attributed to the crossing avoidance of wide roads ^[41][42][118,123]. Husby (2016) conducted a rare study demonstrating a negative relationship between vehicle speed and the WVCs of birds, and the interpretation was that the sound of high-speed vehicles might alert birds earlier or that these vehicles might hit birds harder and throw them further off roads without detection ^[43][139].

Raised roads or roadsides with high embankments decrease the risk of WVCs of many species ^[31][44][45][68,104,128], but this may not be the case for birds ^[46][83]. Lao et al. (2011) and Valero et al. (2015) found that an increased shoulder width increases the likelihood of WVCs of some species ^[47][48][109,119]. Vegetated medians increase the WVCs of a number of species because they may attract species for food or protection or may reduce the width of the gap for crossing ^[31][44][49][68,128,131]. Even rigid median barriers may enhance the risk of WVCs by trapping wildlife on roads ^[50][113]. Road lights attract some species while repelling others ^[26][62]. Those who are attracted to roads by artificial lights may have a higher risk of WVCs ^{[51][52][53][54]}[110,112,129,130]. Moreover, vehicle headlights may dazzle some nocturnal birds ^[31][46][83,128] or may cause some species to freeze, such as possums (Trichosurus vulpecula) and hedgehogs ^[39][55].

Roadside vegetation can be an attractive habitat to a wide range of wildlife or can act as a corridor for movement. A large number of species prefer to cross roads at sites hidden by vegetation cover, which may limit the awareness of drivers or wildlife ^{[15][27][45][55][56][57][58][59]}[35,39,82,99,104,108,115,126]. Actually, the majority of studies show a positive correlation between roadside vegetation and WVCs ^[33][21]. In detail, the structure of roadside vegetation may also have an impact. For example, dense vegetation may force birds to fly higher and decrease the risk of WVCs ^[31][60][106,128]. When incubating birds nesting in low roadside vegetation flush from their nests, they often fly low over the open road and are very vulnerable to WVCs. Galantinbo et al. (2022) found that wood mice (Apodemus sylvaticus) are more likely to cross roads near taller shrubs or after firebreak openings ^[61][132].

Road fences are a very effective mitigation measure to reduce WVCs ^{[62][63][64][65][66][67][68][69]}[40,102,105,111,120,122,124,133]. Moreover, WVCs are likely to be concentrated at fence ends because of the funnel effect of fences ^{[49][62][63][67][70]}[40,102,122,125,131]. Similarly, Cserkész et al. (2013) noted the high rate of WVCs near crossing structures due to fence gaps ^[71][114]. Fences may also become traps for wildlife, resulting from bad design or maintenance ^[72][127]. For instance, some species can climb over or pass through fences and get trapped on roads ^[73][74][107,117].

As mentioned previously, species may improve their crossing ability with time. The road age may correlate with the change in local abundance or the interaction behavior (e.g., habituation) with roads ^[75][72].

Landscape and environmental characteristics affecting WVCs include the surrounding habitat and landscape, topography, weather, time of day, day of the week, and month of the year (Table 3).

The likelihood of WVCs increases when roads intersect suitable habitats ^[41][53][76][118,129,144]. Most studies indicate that the amount and proximity of surrounding habitats (e.g., forest, grassland, wetland, water areas, agricultural land) are positively correlated with WVCs ^{[33][36][59][77][78]}[10,21,126,135,140]. Furthermore, many studies have posited that the diversity of landscapes or habitat types may prompt the movement of wildlife and increase WVCs ^[33][45][58][21,104,108]. However, Puig et al. (2012) found that homogeneous landscapes on both sides of roads could increase WVCs of many medium-sized mammals due to higher crossing attempts, while heterogeneous landscapes were found to reduce WVCs ^[79][146]. Several studies indicate that the presence of national parks nearby raises the risk of WVCs ^[33][21], which is due to high species abundance in protected areas ^[13][80] or high traffic caused by tourism ^[80][7]. There is no major consensus on the correlation between developed areas and the risk of WVCs ^[33][21]. The explanations supporting a positive correlation include a higher abundance of some species attracted by exploiting anthropogenic resources, more nervous behavior, higher traffic density, and lower driver awareness in developed areas ^{[13][30][37][54][81][82][83][84]}[60,80,130,136,141,142,145,147]. The explanations supporting a negative correlation mainly include the low density of some species in built areas due to the avoidance of human disturbance ^[73][76][85][107,143,144]. When linear topographies or landscapes (e.g., riparian structures, ditches, drainages, slopes, ridges) funnel wildlife to roads, they may increase the risk of WVCs ^[36][86][135,148]. The relationships between the slope or height of the surrounding terrain and WVCs seem to be complex ^{[33][55][87][88]}[21,35,57,149].

Various climatic factors concerning WVCs have been discussed in relevant studies, including the temperature, humidity, precipitation, snow cover, wind, barometric pressure, fog, drought, extreme weather, photoperiod, and moon phase (e.g., Refs. ^{[3][29][52][89][90][91][92][93][94][95]}[88,103,112,150,151,152,153,154,155,156]). For instance, Dussault et al. (2006) found more WVCs of moose during days with high temperatures and atmospheric pressure, possibly due to increased nocturnal activity or seeking open areas to avoid biting insects ^[89][150].

The seasonal distribution and the time distribution of the day of WVCs are mainly influenced by the activity patterns of wildlife. The day distribution of the week of WVCs is mainly influenced by the traffic volume. A large number of studies have demonstrated that seasonal peaks of road crossing and WVCs are consistent with seasonal life-history patterns ^{[21][37][44][54][90][96][97][98][99]}[9,68,93,130,136,151,157,159,160]. Furthermore, numerous studies have found that the WVCs of some wildlife, such as ungulates and red foxes, are higher in dark periods because these species are more active during the night, and the WVCs are intensified by poor driver visibility ^{[21][89][98][100][101][102]}[93,150,158,159,162,163]. Many studies have found that WVCs are more frequent on weekends due to higher traffic ^{[21][30][89][98]}[60,93,150,159]. Similarly, WVCs can be more numerous on holidays because of a higher traffic volume ^[103][161].

The driver-related factors affecting WVCs include visibility, attention, reaction, and attitude (Table 4). Driver-related factors usually correspond to some other factors. For example, the visibility of drivers can be affected by the body size and color of wildlife, the driving speed, the road curvature, roadside vegetation, the weather, and the time of day. Moreover, the attention of drivers may decrease on straight roads ^[104][164] or at certain periods of time. The reaction of drivers is related to their visibility, attention, driving skill, driving speed, vehicle type, and the predictability of the crossing behavior of wildlife. The attitudes of drivers toward WVCs may vary with the species, and drivers may even hit certain species (e.g., snakes) intentionally ^{[105][106][107][108]}[165,166,167,168]. Some human activities in certain periods of the year, such as hunting, may promote the activity of some wildlife and increase the risk of WVCs ^{[30][53][77][98]}[10,60,129,159].