This review summarizes the effects of roads and traffic on animal abundance and distribution, based on 79 studies involving 131 species and 30 species groups. Overall, negative effects of roads on animal abundance outnumbered positive effects by a factor of 5 (114 negative, 22 positive, 56 no effect). Amphibians and reptiles showed mainly negative effects, while birds had mostly negative or no effects, with some small birds and vultures showing positive effects. Small mammals generally showed positive or no effects, mid-sized mammals showed negative or no effects, and large mammals showed predominantly negative effects. The study developed predictions for the conditions leading to negative, positive, or no effects of roads on animal abundance. Four species types are predicted to respond negatively: (i) species attracted to roads but unable to avoid cars; (ii) species with large movement ranges, low reproductive rates, and low natural densities; (iii) small animals not limited by road-affected predators that avoid roads due to traffic disturbance; and (iv) small animals not limited by road-affected predators that avoid roads or traffic disturbance but cannot avoid oncoming cars. Two species types are predicted to respond positively: (i) species attracted to roads for a resource and able to avoid cars; and (ii) species not avoiding traffic disturbance but avoiding roads, with predators negatively affected by roads. The study highlights the need for further research but argues that the evidence for population-level effects of roads and traffic is strong enough to warrant routine consideration of mitigation in road construction and maintenance projects. The synthesis suggests that appropriate mitigation depends on whether species are affected mainly through road mortality or traffic disturbance. Fencing and wildlife crossings can mitigate road effects for species affected mainly through mortality, while reducing road and traffic density can mitigate effects for species affected mainly through traffic disturbance. The study also emphasizes the importance of further research on other questions in road ecology, such as the relative importance of road effects versus other impacts on population persistence.This review summarizes the effects of roads and traffic on animal abundance and distribution, based on 79 studies involving 131 species and 30 species groups. Overall, negative effects of roads on animal abundance outnumbered positive effects by a factor of 5 (114 negative, 22 positive, 56 no effect). Amphibians and reptiles showed mainly negative effects, while birds had mostly negative or no effects, with some small birds and vultures showing positive effects. Small mammals generally showed positive or no effects, mid-sized mammals showed negative or no effects, and large mammals showed predominantly negative effects. The study developed predictions for the conditions leading to negative, positive, or no effects of roads on animal abundance. Four species types are predicted to respond negatively: (i) species attracted to roads but unable to avoid cars; (ii) species with large movement ranges, low reproductive rates, and low natural densities; (iii) small animals not limited by road-affected predators that avoid roads due to traffic disturbance; and (iv) small animals not limited by road-affected predators that avoid roads or traffic disturbance but cannot avoid oncoming cars. Two species types are predicted to respond positively: (i) species attracted to roads for a resource and able to avoid cars; and (ii) species not avoiding traffic disturbance but avoiding roads, with predators negatively affected by roads. The study highlights the need for further research but argues that the evidence for population-level effects of roads and traffic is strong enough to warrant routine consideration of mitigation in road construction and maintenance projects. The synthesis suggests that appropriate mitigation depends on whether species are affected mainly through road mortality or traffic disturbance. Fencing and wildlife crossings can mitigate road effects for species affected mainly through mortality, while reducing road and traffic density can mitigate effects for species affected mainly through traffic disturbance. The study also emphasizes the importance of further research on other questions in road ecology, such as the relative importance of road effects versus other impacts on population persistence.