Roads and highways can affect populations of animals directly (e.g. due to road mortality) and indirectly (e.g. due to fragmentation of habitat and proliferation of non-native or predatory species). We investigated the effect of roads on threatened desert tortoise (Gopherus agassizii) populations in the Mojave Desert, California, and attempted to determine the width of the road-effect zone by counting tortoise signs along transects at 0, 400, 800, and 1600 m from the edge of a highway. Mean sign count was 0.2/km at 0 m, 4.2/km at 400 m, 5.7/km at 800 m, and 5.4/km at 1600 m from the highway edge. The differences between all pairs of distances, except 800 and 1600 m, were statistically significant, suggesting that tortoise populations in our study area are depressed in a zone extending at least 400 m from roadways. We speculate that the major cause for this depression zone is road mortality.