Wildlife-vehicle collisions with large animals pose a serious and increasing threat to wildlife populations and contribute to human injuries, fatalities, and property loss. However, the most pervasive roadway impacts to wildlife species are the barrier and fragmentation effects that result in partitioned resources and reduced habitat connectivity. These effects are particularly detrimental to isolated, small wildlife populations inhabiting areas with scares, dynamic, site-specific resources that are unevenly distributed across the landscape. Many desert bighorn sheep populations throughout the Southwest are isolated, small (<100) and can go extinct if the number of individuals is less than 50. When roadways are built or reconstructed within desert bighorn sheep habitat, the viability of the local sheep population is often dependent on the Department of Transportation’s ability to design and construct wildlife crossing structures that will effectively convey sheep from one side of the roadway to the other. However, in order to construct an effective crossing structure, designers need to begin with the answer to one basic question: do sheep prefer to go over or under roadways? A recent and on-going collaborative research project being conducted by Arizona and Nevada sheds new light on the answer to that question. While past Arizona research would guide designers to overpass structures, the current collaborative study indicates that underpasses can work too; it all depends on the details. The evolution of those details can be seen in three research projects, the first of which began in 2006. In 2006, the Arizona Game and Fish Department (AZGFD) began a two-year evaluation of three “small” sheep underpass bridges constructed on U.S. 68 and documented few (32) sheep crossings. These 2006 findings led to the 2011 construction of three sheep overpass structures and two larger underpass bridges on a nearby, reconstructed U.S. 93. AZGFD’s five-year evaluation of these structures documented more than 5,800 sheep overpass crossings and less than 500 sheep larger bridge crossings and led to the general interpretation that desert bighorn sheep prefer overpasses. However, in 2018, a new Interstate-11 section of roadway was completed in Nevada and included one sheep overpass and four very large sheep underpass bridges. Although AZGFD’s evaluation of these structures is currently on-going, preliminary findings are informative. The overpass is effective and performing as expected. The large underpass bridges however are surpassing expectations and to date have had more than 15,000 documented sheep crossings; thus, providing a nuanced answer to the question “desert bighorn sheep prefer overpasses . . . or do they?”.