The Literature Analysis to Determine Optimal Wildlife Crossing Structure Size Study (Study) emerged from Colorado Department of Transportation’s (CDOT’s) desire to determine if there is a point of diminishing return of effectiveness based on target species success rates when it comes to sizing highway wildlife passages. This Study’s objectives are to review and analyze existing monitoring data to determine if there are optimum structure dimensions for underpasses and overpasses for mule deer (Odocoileus hemionus), elk (Cervus canadensis), pronghorn (Antilocapra americana), moose (Alces alces) and Canada lynx (Lynx canadensis), particularly the point at which increasing structure sizes may reach a range of diminishing returns relative to cost and predicted increase in successful crossings. The Study results infer recommendations for a repeatable process to analyze effectiveness and diminishing returns in the future when new field studies are performed, new literature and data may be available, or a new species of interest is the subject. This Study identifies gaps in the literature, available data, and study processes that challenge the effective realization of diminishing return determinations in relation to success rates and highway wildlife passage dimensions. This Study’s results, using regression modeling, may inform development and sizing of highway wildlife passages relative to defining success criteria for larger wildlife and reducing wildlife-related vehicle collisions across Colorado. The results indicate that, given a statistically valid sample size, modeling can be done to determine which structure dimensions (length, width, and height) most strongly influence a species’ (such as mule deer) success rate through wildlife underpass crossing structures. Given this analysis, modeling to predict success rates for a given species and a range of structure dimensions can be generated. It is also possible to determine if a given species has a preference regarding underpass type (bridges or culverts). It is critical that monitoring of wildlife crossings be done to determine success and repel rates because this data will allow further application of predictive modeling for other species. In addition, the project team recommends that success criteria for wildlife mitigation projects be clearly defined and measures identified to determine whether they have been achieved.