Funds for mitigating impacts of roads and traffic on wildlife populations are limited. It is therefore important to determine the relative importance of different road impacts, and thereby the relative wildlife conservation value of different kinds of mitigation. Two such road impacts are the direct effect of mortality (road kill) on animal population size, and the effect of roads as barriers to animal movement, limiting their access to resources. Which of these has the larger impact on wildlife populations, and so where should we place our emphasis in mitigating road impacts on wildlife? To answer these questions we must identify the groups of species whose populations are most reduced by roads and traffic, and identify the primary mechanism responsible for the impacts of roads and traffic on these most sensitive species groups. A meta-analysis of effects of roads and traffic on populations of 312 species identified two groups whose populations are most strongly reduced by roads: (i) amphibians and reptiles, and (ii) mammals with low reproductive rates. Thus, measures to mitigate road impacts should be aimed mainly at mitigating effects on these most sensitive groups. The behavioural attributes of amphibians and reptiles, and the low reproductive rate of the sensitive mammalian species, both suggest that road-kill mortality is likely the dominant mechanism resulting in the heightened sensitivity of these species groups to roads and traffic. The question is then, how best to reduce road kill? A meta-analysis revealed that ecopassages themselves do not reduce wildlife mortality on roads, but fencing does. Therefore, the primary emphasis for mitigation should be to reduce road kill of the most sensitive species groups by keeping them off roads, even if these mitigation measures restrict cross-road movements. I describe various means for doing this. I also discuss why road-kill hotspots are usually not reliable indicators of where these mitigation measures should be installed. Rather, they should be installed where high-traffic roads pass through or adjacent to the natural habitats of the sensitive species groups, irrespective of current road-kill numbers