Construction of wildlife crossing structures (WCS) in the western US, especially overpasses has accelerated dramatically over the past decade. And whereas WCS applications during the previous decade typically were associated with major new and reconstructed highway projects (Trans-Canada Highway, SR 260 and US 93 in Arizona, US 93 in Montana), more recent applications have been a mix of major construction and stand-alone projects (retrofitting) on existing highways. The multiple benefits (ecological, budgetary, technical, political) of recent WCS applications will be considered with an overview of WCS projects implemented across the West. The “traditional” measure of WCS success in terms of their effectiveness in promoting wildlife passage, diminishing fragmentation, and reducing genetic isolation, and accommodating species preferences for WCS type (overpass vs. underpass) will be considered. However, recent retrofitting projects point to benefits that transcend these ecological benefits.
Existing highway retrofitting with stand-alone or drop-in WCS projects has greatly expanded the scope of opportunities to address wildlife-vehicle collisions (WVC) and ecological connectivity, especially as transportation budgets declined. Even with substantially greater funding commitment to highway construction, the need for retrofitting of existing highways will remain high. Prefabricated WCS designs (e.g., large “super box” culverts, precast-concrete and metal-plate arches) present a cost-effective and readily implementable (e.g., quick) means to retrofit existing highways with minimal disruption to traffic (thus reducing political resistance), to the point that they are increasingly being done on interstate highways. The fact that overpasses can be constructed across the gamut of terrain conditions, from flat to mountainous, coupled with the fact that foundation/preparatory work can be done off the roadway in advance of rapid WCS installation (minimizing traffic disruption) has the potential to revolutionize the programmatic implementation of WCS in priority areas.