Desert tortoise road mortality in Mojave National Preserve, California

Direct impact by motor vehicles is a known threat to the federally- and California state-listed Agassiz's desert tortoise (Gopherus agassizii). Tortoise sign adjacent to paved roads is depressed, indicating that roads can have a population-level effect extending up to 400 m away from the road edge. We found that temporarily erected warning signs equipped with flashing lights had no effect on driver awareness of a potential tortoise on the road and that population depression adjacent to roads with lower speeds and greater driver awareness was similar to roads with heavier traffic traveling at higher speeds and lower driver awareness. The management implications are that slowing traffic with lower posted speed limits and increased law enforcement and improving driver awareness may not reduce impacts to tortoise populations.