Collisions between wildlife and vehicles are detrimental to both wildlife and human safety. A variety of mitigation methods have been deployed with the intent of increasing ungulate awareness of approaching vehicles. Wildlife warning reflectors are one such method. Roadside reflectors are designed to reflect headlights into the right-of-way and alert ungulates to approaching vehicles. Studies of the effectiveness of these reflectors have yielded mixed results. We conducted a robust test of reflectors in central Wyoming, USA, during 2013 and 2014, and unexpectedly discovered a potentially more effective method than the reflectors for reducing collisions between vehicles and mule deer (Odocoileus hemionus). In our initial experiment, we manipulated 10 1.6-km segments of highway by leaving their reflectors exposed or covering them with a white canvas bag with the intention of neutralizing the reflector. The treatment configuration was swapped monthly. We counted deer carcasses under each treatment for 1 year and observed deer roadcrossing behavior using thermal video cameras. Carcass rates were 33% less in the white canvas treatment relative to uncovered reflectors. Deer in the white canvas treatment also stopped before entering the road 20% more often, ran into the road from the right-of-way 11% less often, and fled from the road 12% more often than when reflectors were uncovered. In a follow-up experiment, we found that deer carcass rates were 32% less when reflectors were exposed versus covered with black canvas. We further found that deer road-crossing behavior was least risky in a white canvas treatment, intermediate in a reflectors treatment, and most risky when reflectors were removed from posts. Taken together, these results indicate that, although reflectors were moderately effective, white canvas was substantially more effective in reducing deer-vehicle collisions. This unexpected finding suggests that new vigilance-enhancing mitigation methods should be explored as a way to reduce wildlife-vehicle collisions. Ó