The Basic Deer Underpass

This article summarizes the features needed in an underpass designed primarily for either white-tailed or mule deer. Each location has specific needs, and this summary leaves out many qualifying details that help ensure the success of any crossing. Several of the Wildlife Crossings Toolkit Database accounts describe features to consider when designing an underpass for deer. An ideal ecological solution considers all the species and ecological processes in an area, and many other species requiring connectivity will be left out if only deer are considered.

Minimum Features

Consider the adjacent land use and habitat condition on both sides of any crossing structure. Will a structure lead into a new subdivision or off-road vehicle playground now or in the future? If so, choose another location. Effective structures connect suitable habitat on one side of a highway with suitable habitat on the other side, with at least minimal habitat in the structure vicinity.

Underpass minimum dimensions are 8 ft x 24 ft with an openness ratio of 0.9. As the number of traffic lanes increases, an underpass increases in length to span all lanes. From an animal’s perspective, the opening at the far end of an underpass appears smaller as the length increases, and the small aperture appearance is intimidating. As an underpass increases in length, proportionately increase the height and width. The openness ratio helps to make the aperture appearance large enough so that animals consider it safe to enter. The openness ratio is defined as height x width/length.

If the underpass also conveys water, provide an area of unsubmerged land as a dry walkway. Deer will readily use shelves constructed under bridges or culverts.

Barrier fencing is necessary to divert deer from crossing at grade instead of going through the underpass. Construct barrier fencing a minimum of 8 ft high. Fencing must be maintained to avoid deer finding and using holes in the fence. Fencing must touch the ground or be buried to avoid deer or other animals tunneling under it. Wire mesh works well.

Consider the ends of the barrier fencing to reduce the opportunities for ‘end-runs’ around the end of the fence. This can be done by extending the fence into unsuitable habitat, or by designing the ends as funnels. Several designs are shown in the Wildlife Crossings Toolkit.

No barrier fence is 100% effective, although some come close. Use Escape structures such as ramps to allow deer that make their way onto the roadway to escape off the road.

Better

Use natural substrate on the bottom of the underpass. Bridges are better than culverts, and bottomless culverts are better than box or continuous culverts.

Large headroom volume increases a deer’s comfort level in a strange environment such as an underpass, which has no natural equivalent. Underpasses in a V-shape instead of an inverted U-shape provide the most open feeling. However, filling in the lower part of an arch culvert to achieve a V-shape would reduce the total volume of the structure and would not be desirable. A small amount of natural material such as stumps and branches at the edge of the sidewalls provides cover for smaller animals to use the underpass.

In the absence of site-specific knowledge of local deer movements, drainages and ridges are good general locations for crossings. An underpass (bridge or culvert) would be most topographically suitable at a drainage. A drainage does not have to contain water to function as a travel corridor.

If the underpass is used in a 4-lane highway or greater, consider a separated median with two shorter underpasses. This reduces the size necessary and increases the amount of light in the center. Fence the median around the opening of the two underpasses to avoid animals entering the highway from the median. Short vegetation in the median is beneficial.

Best

Situate the crossing so that it connects as broad an area of suitable habitat as possible, for the entire useful life of the structure. Many animals use crossing structures less when human activity occurs nearby, so try to design them away from known or planned human use.

Situate the underpass in an area of known habitat and crossings. This will usually be near a drainage or ridge, and depends on the habits of the deer herd in question. Migratory herds usually have defined areas or routes that can be identified by consistent roadkills or crossing information.

Consider the sights and sounds deer will encounter as they approach the highway. Encourage deer to approach and enter the underpass by reducing traffic sights and sounds. Jersey barriers or “Texas” barriers (larger Jersey barriers) on top of an underpass can provide protection for vehicles exiting the roadway and produce a sight and sound barrier at the same time. If vegetation is used as a visual barrier, use unpalatable species to avoid attracting animals (not just deer) to the side of the road; vegetation does not provide substantial sound protection.

Roughly texture the interior surface of an underpass to reduce unnatural sounds while animals travel through the enclosed area. Some commercial products are available to line culverts. Avoid products that may leave a petroleum odor.

Baiting the underpass for a few seasons with an attractive food such as apples or grain has been used to create habitual use. Once use has been established, baiting is no longer necessary.

Underpasses for deer are among the easiest to design, because deer are adaptable species. Consider other animals in addition to deer. While ungulates cause the largest amount of vehicle damage and are the most obvious roadkill victims, highways adversely affect most other species too. With a little forethought, other species can be included in the design criteria rather easily and often without much additional expense. Scan through the Wildlife Crossings Toolkit for numerous other ideas to encourage animals to safely coexist with highways, including:

 

• Construct bat ‘abodes’ on the undersurface of underpasses and bridges for roosting bats
• Place vegetative debris such as old stumps, logs and small brush along one edge of the bottom of an underpass for a foot or so as cover for small mammals and amphibians
• Include the largest amount of riparian habitat as possible under a constructed bridge to allow for travel along the water’s edge. Riparian areas with dense vegetation are favored by many species for travel corridors.

--Sandra Jacobson
July 17, 2002