This paper reviews the empirical literature on the effects of roads and traffic on animal abundance and distribution, aiming to provide an up-to-date summary of the state of knowledge. The review includes 79 studies with results for 131 species and 30 species groups, covering a wide range of taxa, trophic levels, and habitats. Overall, the number of documented negative effects of roads on animal abundance outnumbered positive effects by a factor of 5, with 114 negative responses, 22 positive responses, and 56 showing no effect. Amphibians and reptiles tended to show negative effects, while birds showed mainly negative or no effects, with a few positive effects for small birds and vultures. Small mammals generally showed either positive effects or no effect, mid-sized mammals showed either negative effects or no effect, and large mammals showed predominantly negative effects. The paper synthesizes the information to develop predictions of conditions leading to negative, positive, or no effects of roads on animal abundance. Four species types are predicted to respond negatively to roads: (1) species attracted to roads and unable to avoid cars, (2) species with large movement ranges, low reproductive rates, and low natural densities, (3) small animals avoiding habitat near roads due to traffic disturbance, and (4) small animals unable to avoid oncoming cars. Two species types are predicted to respond positively to roads: (1) species attracted to roads for resources and able to avoid cars, and (2) species not avoiding traffic disturbance but avoiding roads, with their main predators showing negative population-level responses to roads. The paper also discusses the limitations of existing studies and identifies areas where further research is needed. Despite the strong evidence for population-level effects of roads, more well-designed studies are required to fully understand these impacts. The authors argue that the current evidence is sufficient to merit routine consideration of mitigation measures in road construction and maintenance projects.This paper reviews the empirical literature on the effects of roads and traffic on animal abundance and distribution, aiming to provide an up-to-date summary of the state of knowledge. The review includes 79 studies with results for 131 species and 30 species groups, covering a wide range of taxa, trophic levels, and habitats. Overall, the number of documented negative effects of roads on animal abundance outnumbered positive effects by a factor of 5, with 114 negative responses, 22 positive responses, and 56 showing no effect. Amphibians and reptiles tended to show negative effects, while birds showed mainly negative or no effects, with a few positive effects for small birds and vultures. Small mammals generally showed either positive effects or no effect, mid-sized mammals showed either negative effects or no effect, and large mammals showed predominantly negative effects. The paper synthesizes the information to develop predictions of conditions leading to negative, positive, or no effects of roads on animal abundance. Four species types are predicted to respond negatively to roads: (1) species attracted to roads and unable to avoid cars, (2) species with large movement ranges, low reproductive rates, and low natural densities, (3) small animals avoiding habitat near roads due to traffic disturbance, and (4) small animals unable to avoid oncoming cars. Two species types are predicted to respond positively to roads: (1) species attracted to roads for resources and able to avoid cars, and (2) species not avoiding traffic disturbance but avoiding roads, with their main predators showing negative population-level responses to roads. The paper also discusses the limitations of existing studies and identifies areas where further research is needed. Despite the strong evidence for population-level effects of roads, more well-designed studies are required to fully understand these impacts. The authors argue that the current evidence is sufficient to merit routine consideration of mitigation measures in road construction and maintenance projects.