Wildlife–vehicle collisions pose a major safety concern to motorists and can be a significant source of mortality for wildlife. Additionally, roadways can impede movements and reduce habitat connectivity. For migratory ungulates, these problems can be exacerbated when roadways bisect migration routes, as is the case in Southwest Wyoming, USA, where a 21-km section of U.S. Highway 30 overlaps with a critical winter range and migration route used by thousands of mule deer (Odocoileus hemionus). In an effort to reduce deer–vehicle collisions (DVC) and maintain migratory connectivity, the Wyoming Department of Transportation installed 7 concrete box-culvert underpasses with continuous game-proof fencing between each crossing structure. To evaluate the effectiveness of this mitigation project, we used remote cameras to quantify the number of mule deer that used the underpasses, estimate passage rates through time, and compare rates of DVCs before and after underpass construction. Through 3 years of monitoring (which encompassed autumn migration [2008, 2009, and 2010], winter use, and spring migration [2009, 2010, and 2011] for 3 migration cycles), we documented 49,146 mule deer move through the underpasses. Passage rates of deer approaching underpasses steadily increased from 54% in Year 1 to 92% in Year 3. Peak movements during the autumn migration occurred in mid-December, while peak spring movements were in mid-March and early May. Underpass and fence installation effectively reduced DVCs by 81%. Had fence gates remained closed and cattle guards clear of snow, DVCs could be eliminated altogether. Our results suggest that underpasses, combined with game-proof fencing, can improve highway safety for motorists while providing safe and effective movement corridors for large populations of migratory mule deer. © 2012 The Wildlife Society.