Understanding Appropriate Approach Slopes to Wildlife Highway Overpasses

Focus Area

Wildlife & Ecosystems


Natural Resources






1-2 years

Research Idea Scope

The optimal slope of approaches to highway overpasses for wildlife does not appear to be well documented. Understanding slope constraints could inform the design of overpass structures along the nation’s roadways. Cost constraints and space limitations in some transportation departments’ rights of way could necessitate the construction of overpasses with steeper approaches. Meanwhile, constructing overpasses can be costly, and transportation departments must ensure constructed wildlife passageways are effective. The goal of this research is to document the range of slopes to crossing approaches and the associated suite of species that utilize constructed crossings in North America. The objectives of this project include (1) documenting known species’ maximum habitat slope preferences (bighorn sheep have likely been studied); (2) documenting the range of slopes of constructed wildlife overpasses; (3) identifying the focal species of constructed overpasses; and (4) describing habitat characteristics at project sites such as: • Line of sight to the crossing and across the crossing • Characteristics of the crossing entrance/exit (open vs. vegetated, escape cover elements, etc.) • Characteristics of adjacent lands and the crossing connection (ridge to ridge, upslope to downslope, at grade, etc.) • Fencing configurations to the crossing entrance/exit With this research, other factors should be considered, including documenting predator avoidance and escape cover strategies and species movement preferences (riparian corridors, along contours, etc.). An additional component of this research could include an investigation on understanding the optimal and maximum appropriate slopes of jump-outs (or escape ramps).

Urgency and Payoff

This research could result in the production of a guidebook that contains readily accessible design criteria that would benefit practitioners and inform project development nationwide. Understanding the optimal slope of overpass approaches would be most beneficial if completed within the next 2 to 3 years, particularly as a large overcrossing project in southern California moves through the planning stages.

Suggested By

Lindsay Vivian Caltrans (916) 653-8566

[email protected]