#49 Wildlife Crossing Permeability
- Duc Phan
- 6 days ago
- 2 min read
In our previous brief on Wildlife crossings, some treatments were identified that can help to protect animals from vehicle strikes on roads. These include various types of underpass and overpass, bridges, ladders and perimeter fencing, common versions of which are outlined under Fauna Management in the Austroads Guide to Road Design Part 6B[1]. Treatment selection should reflect the types of fauna at risk and the ways the animals move, and should aim to provide continuous habitat linkage where possible.
Also impacting the movement and safety of animals on and around roads is the presence of road safety barriers designed to protect road users. With relatively little known regarding quantified and generalisable impacts, a recent report from the United States examined the effects of different median barrier designs and permeability on wildlife vehicle collisions (WVC)[2]. The study compared the effects of median concrete, cable (wire rope) and steel beam guardrails/barriers on California highways. Also considered was the type of median (vegetated, paved, with/without barrier) as a predictor of WVC rates, particularly involving smaller animals. A key finding of the research included that:
“Fewer wildlife enter roadways and are killed in the presence of constructed median types than undeveloped types. Although this may result in a reduction in WVC, it also results in a reduction in wildlife permeability”.
The report also notes that this finding contrasts with some other studies, highlighting the impact of many context-specific variables in terms of species, study sites and treatments.
Comparing the three median barrier types for animals of different sizes, the following findings were reported:
Greatest and least permeability may be provided by cable barriers and concrete barriers, respectively.
“For a medium-sized animal (coyote), the presence of a metal beam guardrail, or undeveloped median was a significant predictor of WVC”.
“For a large animal (mule deer), we found that the presence of a metal beam guardrail, concrete barrier, or undeveloped median were predictors of WVC”.
Considerations
As these outcomes suggest, a major challenge is to achieve an appropriate balance of safety and habitat connectivity, and consideration of site-specific factors is critical to maximising treatment benefits. Even seemingly innocuous modifications to existing infrastructure may have adverse and unanticipated impacts. For example, there has been some concern expressed in Australia that motorcycle underrun protection on W-beam guardrails may limit the movement of terrestrial fauna such as koalas, wombats and echidnas. A potential measure to address this could be to provide gaps in the underrun barrier at appropriate spacings. However, given the lack of published research on this specific issue, as well as others concerning WVC, further work is needed to properly identify problems and solutions.
[2] Haworth, L., et al. (2025). Wildlife Connectivity and Which Median Barrier Designs Provide the Most Effective Permeability for Wildlife Crossings. University of California, Davis.
