Step 2: Identify Solutions & Options

2.1 Select General Mitigation Type

2.1.1 Identify Species to Benefit from Potential Mitigation

In this step, the user examines the species selected in Step 1.2.4 (the single or multiple species most likely to benefit from crossing measures) and begins the process of determining the most effective type of crossings or other mitigation. It is possible to design mitigation strategies for multiple species use. Not every crossing will allow all species present to pass, but to the extent that several species can use a crossing, the objective of creating landscape permeability is closer to being achieved.

We presented in Table 1.2.4.4, a summary of animal needs for mitigation based on their movement, habitat, and biological issues related to the road and traffic. A simple starting point to assess whether a species or wildlife type needs the assistance of crossing structures is to ask one of the following two questions:

1. If this is an aquatic culvert/bridge, are the flow characteristics of the stream (e.g., timing of flow, velocity, depth, seasonality) being simulated in the passage in ways similar enough to natural conditions that they allow both young of the year and adults to pass upstream at critical times and flows?
2. If the concern is terrestrial wildlife, can the species move across the roaded areas without avoidance behaviors or, in the case of humidity sensitive amphibians, without desiccation? Importantly, is the average daily traffic flow measured in the hundreds of vehicles or less?

If you answer yes to either question 1 or 2, then there may not be a great need for mitigation measures at this time. Regardless, the situation may necessitate further investigation into species' abilities to move across the roaded landscape regardless of traffic. Future projections of traffic volumes and their putative effects on species movement are important considerations. In specific situations, it will be important to track developments in research that focuses on how the species react to the roadway and traffic. Understanding species natural history characteristics will be beneficial.

The process we use here in step 2.1.1 starts with classifying the size of animal and its trophic level (i.e., carnivore or herbivore) and considers its size and mode of movement. We give generalities as to what general type of crossing may be best for these types. In section 2.1.2 the different wildlife crossings types are further described so the user is better able to determine what may work best for the given situation.

Herbivores

Herbivores are plant eaters. The herbivores of most concern for safety reasons are deer, elk, moose, bison, antelope, and bighorn sheep. These animals are prey for carnivorous species (meat eating species) and generally respond with caution to underpasses that are tunnel-like or appear dark and closed. Additionally, there are differences in the 'willingness' of the different species of ungulates to use underpasses. For example, mule deer appear to use box culverts with minimum measurements of 5.5 m high by 18.3 m across. Elk and moose generally appear much less willing to do so. Work by Clevenger in Canada suggests that elk, moose, and deer tend to use overpasses much more willingly than underpasses. This does not mean that underpasses will not be used by ungulates, but what is clear is that the more open and large the underpass, the more likely ungulates will use it. Human presence in passages, in the form of frequent disturbance (vehicles, ATVs, walking) impacts the use of crossing structures by animals. Underpasses that might be used more frequently by ungulates will be more likely to be avoided with increasing human disturbance.

Carnivores

Carnivores, or meat eating animals, are more likely to use underpasses smaller than those typically used by ungulates. Intermediate-sized carnivores, such as coyotes, bobcats, and wolves appear to use underpasses willingly, as do black bear. Again, with increased human presence or disturbance, use in impacted. Larger carnivores, such as grizzly bears typically use both overpasses and underpasses in Canada.

Understanding the differences in the natural history and preferences of herbivores and carnivores will help to inform the selection of different passages that are effective for different species such as predators and prey, and will aid in the design of multi-species passages that may meet many of the requirements of both. It is a non-trivial task to design a passage for predator and prey, but this has been done with two overpasses and select open underpasses in Banff National Park, Alberta, where grizzly bear, puma, and wolf have been photographed using the same areas as elk, moose, and mule deer (Clevenger and Waltho 2000, 2005).

Amphibians and Reptiles

Amphibians and reptiles, commonly referred to as 'herps' present their own challenges to passage, regardless of their roles as herbivores and carnivores. Amphibians need:

1. to be able to find a passage in a matter of meters from the point at which they are intercepted by the fences that guide them to passages or the places they are moving from, such as an upland or pond;
2. specific drift fencing or walls to 'guide' them to the passage;
3. moist natural floor conditions along the passage to ensure they do not dry out along the passage;
4. moist conditions that are not contaminated with vehicle run-off that could be absorbed through the skin; and
5. light in the passage to help them see it leads to another area. Reptiles need similar passage requirements, although they are not limited by moisture concerns. See Scott Jackson's summary of amphibian and reptile crossings at: http://www.umass.edu/nrec/onlinedocs.html.

We know of no across-the-road walkways or overpasses have been built for these species. Overall, herps require much smaller passages under the roadway when compared to most other species. It is also worth noting that reptiles in particular may be subject to intentional killing by motorists as they cross the road. Ashley et al. (2007) conducted an experiment in Ontario with two wildlife decoys, (a plastic snake and a plastic turtle), and a disposable cup. Each was set individually on the yellow line between opposing lanes of traffic. While sitting concealed in the bushes nearby, the investigators observed motorists reactions to the three treatments (turtle, snake, and cup). Motorist response was documented as hit, miss, or rescue (some motorists stopped to pick up the turtle). Using log-linear analysis they found evidence that reptile decoys were hit at a higher rate than by chance with approximately 2.7% of motorists intentionally hitting them. This experimental situation was conducted on a site bordered by a United Nations World Biosphere Reserve Program wetland. The results of this study are mentioned here because if the results are extrapolated to other locations, then approximately 2.7% of drivers may be inclined to intentionally kill a turtle or snake on a road. If the relationship holds, and the volume of traffic on a specific road is multiplied by 2.7%, then one can obtain an average number of drivers per hour that may be inclined to intentionally hit these animals. Whether turtle and snake populations can sustain this type of mortality on roads would need to be investigated. These results suggest that installing reptile underpasses is a much more feasible and effective solution than erecting signs to warn motorists to slow for these creatures.

Small and Medium-Sized Mammals

Small and medium-sized mammals have been documented using underpasses of all shapes and sizes across North America. These animals are also further classified as predator and prey, so some of the same initial considerations with herbivores and carnivores in general need to taken into account. For instance, coyotes and fox may pass through a culvert without any vegetation or cover more readily than a rabbit or small mammal. This is more of a generality, not a fast rule. Cotton-tailed rabbits and small mammals have been photographed traversing long culverts with no cover in Florida (Dodd et al. 2004) and Montana (Foresman 2003). Current studies are exploring the use by small mammals of underpasses with added cover in the form of tree stumps to determine if use is increased (Bellis 2007). Foresman (2003) has that found small and intermediate-sized mammal use of existing culverts with water present if a grated metal shelf is placed in the culvert.

Flying Animals

To date, the only known bird passage in North America has been built in Arizona's Pima County for Burrowing Owls. The 'passage' is comprised of berms and vegetation built along the sides of the highway and in the median to facilitate flight over the flow of traffic.

Pima County Planner Rick Ellis in Front of median with soil fill and trees planted to intercept flight of pygmy owl, which was present along route of this highway. The owls have since died out and the remaining male was brought into a captive breeding program.

This approach to changing the flight path of flying organisms is also being explored and created for endangered species of insects. Examples include Oregon's efforts to change air flow over a highway to help the Checkerspot butterfly, and Taiwan's protective netting and lane closures for the mass migrations of a purple milk weed butterfly (http://news.bbc.co.uk/1/hi/world/asia-pacific/6491255.stm). There is precedence in the United States for slowing speed limits in order to protect insects. In eastern Washington, the alkali bee is an important pollinator for the alfalfa crops that are bisected by county roads. The county commissioners agreed to reduce the speed limit to 20 mph during the seasonal daylight hours when the bees are traversing the road. See picture.

Underpasses have been documented being used by wading birds and birds that have a tendency to walk across the landscape as well (see photos below).

Arboreal Animals

Australia is the leader in creating rope and metal bridges for animals restricted to trees (arboreal). This is a relatively new technology, but initial studies have shown they worked in North Queensland. See the article Bridges to slow possum deaths.

Aquatic Animals

Fish and aquatic species passage is beyond the specific scope of this decision guide, however, the use of multiple-species passages that pass both aquatic and terrestrial species is beneficial and cost-effective. In fact, many state departments of transportation are asking how these multiple passage types can be combined into one structure. If inclined to seek details on necessities of aquatic crossings, we refer the user to the overall North American Fish Crossings website (http://stream.fs.fed.us/fishxing/index.html), or their specific state or province guidelines. If the user is interested in including terrestrial passage in an aquatic structure, this decision guide can be of assistance. S. Jackson also summarizes needs for terrestrial and aquatic crossings at: http://www.umass.edu/nrec/onlinedocs.html. The USDA Forest Service has completed a guide to riparian road restoration. It helps readers understand the necessary steps for replacing older culverts with functioning ones. The guide can be found at: http://www.fs.fed.us/rm/pubs/rmrs_gtr102.html.

2.1.2 Identify Ecological Processes (Water Flow, Animal Movement, Other)

Ecological processes need to be accommodated for in transportation corridors, and the installation of wildlife crossings is an opportunity to take into consideration naturally flowing processes, such as the flow of water, the movement of sediment, and possible disturbance regimes such as avalanches, mud flows, and fire. Consultation with natural resource maps, wetlands maps, and hazard maps, can help find the pathways of natural processes that flow through the area. It is also helpful to contact local biological and geological professionals to learn more about these natural flows. The most typical natural process that is accommodated within the road framework is the flow of water. In the case of large wetlands and rivers, and sheet flow across the land, viaducts have been constructed and are in the plans for several road projects, including the future upgrades to Interstate 90 in Washington, and the Route 78 pass over the Mississquoi National Wildlife Refuge wetlands in Vermont, which will be a 152 m (500 feet) long bridge, 3 m (10 feet) off the ground. This accommodation for natural processes can allow the maximum permeability possible in a road corridor, because it figuratively lifts the road and traffic up off of the ecosystem so the 'ecological footprint' of the road and traffic is much reduced. A secondary way to accommodate water is to build bridge over the water system, while also leaving some terrestrial upland available under the roadway. In Montana, US 93 upgrades resulted in dozens of new wildlife crossings, many of them accommodating water while incorporating terrestrial wildlife movement.

This bridge along US 93 near the town of Sula accommodates both the Bitteroot river and terrestrial wildlife movement along paths in between the rocks. Photo credit: P.Cramer.

2.1.3 Identify Landscape and Topographic Features That May Affect Movement and Mitigation

An important consideration in the selection of a wildlife crossing is the topographic relief of an area and the landscape features near the road. Wildlife typically prefer to approach a road crossing while following a natural feature such as a ridgeline, low lying riparian area, or a corridor of vegetation where they feel safe. Paying special attention to where these features occur near a potential mitigation site will not only assist in selecting what mitigation type is best suited for the area, but also help to ensure the successful movement of animals through the landscape to the crossing. Topographic features such as deep water or a steep canyon wall may also serve to prevent wildlife from entering the roadway. These areas can be used as possible "guiding" features. Placement of the crossing may be as important as the type of crossing, so the consideration of landscape and topographic features is critical. A field visit is the best way to be able to judge these characteristics of the landscape. The best way to evaluate the placement of crossings (Step 2.2) would involve wildlife crossing data, and a field visit coupled with expert opinion. Placement is further discussed in Step 2.2.

The wildlife passage at the base of the hills was located at the northern edge of a mountain range where mule deer and other wildlife preferred moving, rather than through the open sagebrush lands to the north (background in picture). Although the line of site through this passage is less than optimal (wildlife should be able to see that this passage leads to where they want to go), the mule deer in the area have adapted to it and monitoring has found dozens of deer passages every month under the highway bridges. No other mammals have been photographed using this passage in the first four months of monitoring.

A wildlife passage was created when the plans for this bridge along US 93 in Montana were extended to include upland as well as the river. Wildlife typically use riverine areas for movement pathways, especially in steep terrain as in this picture.

2.1.4 Identify Engineering and Maintenance Concerns

The collaborative work of engineers, planners, and ecologists is critical in this step of the selection process. There are many factors that constrain the selection of the type and placement of crossings. For example, the road grade is an important consideration and may influence the decision to install a bridge or culvert. Soil type and bedrock stability are considerations. The design of a bridged crossing is also dependent on engineering concerns as much as wildlife preferences. While wildlife prefer to cross under bridges that have gradual slope sides (such as those with a 2:1 ratio), engineers need to be considered to see if this slope is possible. Terrestrial wildlife and streams can be accommodated in the same structures but wildlife ecologists need to work with engineers to provide information on wildlife preferences, while engineers need to be consulted on structural issues. There needs to be sufficient upland along a crossing, enough room for the target species to pass under the planned height and openness of the passage, and if rip-rap rocks are placed to stabilize the stream bed, their size needs to be considered so that they could be negotiated by the animals that would use the passage. Through iterative processes, engineers, planners, and ecologists working together can come to a solution that satisfies both engineering and wildlife considerations. If an overpass is selected as the mitigation of choice, landscape topographic features can be used to some degree to minimize costs. McGuire and Morrall's paper (2000), found in section 2.1.8 in References, can be helpful: it describes some engineering concerns that were dealt with in the 24 wildlife passages that were built along the Trans Canada Highway in Banff National Park. Terry Brennan, the USDA Forest Service Highway Program Leader in Arizona has written an article for the Wildlife Crossings Toolkit site, entitled, "The Severn Dwarfs: Often Ignored Highway Project Issues." These issues are all important to the final plans for construction of mitigation. The article can be downloaded at: http://www.wildlifecrossings.info/sa017.htm.

2.1.5 Weigh Cost Concerns with Potential Benefits

The cost of wildlife crossings is probably the number one stated reason why some Departments of Transportation and Ministries of Transportation choose not to build them. For those unfamiliar with transportation project costs, a few statistics will put the cost of wildlife crossings into perspective. The Washington State Department of Transportation asked other states what the average costs were to build one mile of one lane of highway (a single-lane mile). In 2002, 25 state Departments of Transportation reported the cost averaged from $1 million to $8.5 million, with an average cost of $2.3 million (WSDOT 2002, White 2007). White (2007) reported that the American Road and Transportation Builders Association (ARTBA) stated that road construction projects put into place in 2006 totaled $105 billion. Within that context, wildlife crossings can cost from approximately $10,000 for a small culvert, to several million dollars for an overpass. In 1997, 2 wildlife overpasses were built over the Trans Canada Highway. The overpasses each cost $1,851,000 in Canadian dollars, or $1,350,799 U.S. dollars (http://www.mountainnature.com/Articles/CrossingStructures.htm). Consumer Price Index adjusted costs (1 Jan 1997-31 July 2007) would be ~$1,753,000, but may not account for the increased cost of some materials. Costs may be higher in different localities. Colorado is working to raise $4.5 million in federal funding to build an overpass in the vicinity of milepost 188 over I-70 west of Vail Colorado. An overpass was selected by advocates and consulting companies over an underpass in part because of 'engineering constraints, and the fact that a span bridge for wildlife to cross under the road would be cost prohibitive and would create unnecessary traffic delays' (Southern Rockies Ecosystem Project, url: http://www.restoretherockies.org/wildlife_bridge.html) . An estimate from engineers and planners can help weigh the cost of a box culvert versus a bridge, but financial costs are just part of the decision process. To illustrate an example of the struggle of weighing cost concerns, the Utah Department of Transportation is weighing the possibilities of installing an overpass, bridges, and box culverts to pass mule deer and elk across Interstate-70. If the least expensive box culvert is installed in a particular place, there is a much higher chance that the elk will refuse to use it than if it was a bridge underpass. Based on the results of the research project that led to this decision guide, in all of the United States, only 4 elk have ever been recorded in reports and papers, passing through a box culvert. That is after over 30 years of monitoring wildlife crossings and existing box culverts. If the less costly box culvert is chosen, additional considerations include the consequences of elk refusal to use it and possible attempts by elk to compromise the fence in order to cross the road. Certainly costs need to be considered with safety and ecological effectiveness in mind. Step 2.5 of the guide instructs on how to estimate cost effectiveness.

2.1.6 Identify Appropriate General Wildlife Crossing Type

Overpasses

Wildlife overpasses are among the most effective structures for allowing wildlife of many types to move relatively unimpeded across the road or railway. They have been successfully employed in Europe and North America.

Figure 2.1.1. Wildlife overpass (known as the Wolverine Overpass) across the Trans Canada Highway, in Banff National Park, Alberta, Canada. Picture taken approximately 2000, three years after passages were installed. As time progresses, the vegetation on the overpass will grow to resemble the nearby forest. Passage is 50 m wide, 70 m long. Photo credit: K. Gunson.

By early 2007 there were three overpasses in Canada (two in Alberta in Banff National Park, and one in British Columbia over the Okanagan Connector Freeway), and two in the United States that have been constructed exclusively for wildlife passage (one in Utah over Interstate 15 for mule deer, and one in New Jersey over Interstate 78 for white-tailed deer). There are several other overpasses built for mixed uses such as wildlife and human recreation or unpaved roads. In the United States these are found in New Jersey over Interstate 78 along with the exclusive wildlife overpass, and in Florida along the Florida Greenway Trail over Interstate 75. In Canada the two wildlife-exclusive overpasses in Banff National park have the best documentation of wildlife use data .They are 50 meters wide and have been designed to blend in with the natural surroundings with earth, bushes and trees on them (see picture). Since their construction in 1997, they have been monitored with track plates, cameras and wire snags for hair samples for genetic analysis. These crossings have facilitated wildlife movement, in part because the accompanying fences are regularly maintained. Thousands of animal crossings have been documented across these two overpasses, including elk, mule deer, moose, grizzly bear, black bear, and puma (see Clevenger and Waltho 2000, 2005). These are perhaps the best known and best documented overpasses in North America. It is possible that overpasses with less width would be used regularly if fencing was placed along the road right of way and was continually maintained. Bobcat use has been documented over the multi-use overpass along Florida's Greenway has documented bobcat use (see picture). This passage spans a 6 lane divided highway. The two overpasses in Banff and the multi-use passage in Florida are the only three North American overpasses seriously monitored for wildlife use. There is an overpass approximately 7 meters wide in British Columbia over the Okanagan Connector Freeway near Peachland which has anecdotal evidence of mule deer use. The predecessor to the British Columbia overpass is in Utah over Interstate 15 south of the town of Beaver. This is the first overpass built in North America (1975). It is approximately 7 meters wide and has sign of mule deer use (see picture). There is one overpass 15-22m wide over Interstate 78 near Berkley Heights, New Jersey which has anecdotal evidence of use by white-tailed deer and other species. More data is needed on the use of these structures. Specifically, measures of human presence or disturbance may be important determinants of their use. In selecting an appropriate size of overpass for future projects, perhaps a compromise between the wide (50 m) overpasses in Banff, and the narrow (7m-22m) overpasses in Utah and British Columbia may prove to be acceptable to target species while providing cost effectiveness, however it is not possible to determine minimum effective size with available data.

At-Grade Crosswalks

At-Grade Crosswalks are designated areas for wildlife to pass across the roadway. The idea is that motorists will be cautious for wildlife where these at-grade crossings are located. Very few crosswalks have been employed and even fewer studied to see if they reduce the problem of wildlife-vehicle collisions. Lehnert and Bissonette (1997) studied the efficacy of a crosswalk for mule deer and elk in rural Utah on a 4 lane divided highway and on a 2 lane road where fencing led the animals to specific area of roadway to cross. The method used a series of 3 stationary deer warning signs to warn motorists to reduce speed. After installation, deer mortality declined~42% and ~37% along the roads, but they could not demonstrate statistically that the mortality reductions were a result of the crosswalk system, rather data led the researchers to surmise that the decrease in Wildlife-Vehicle Collisions was largely due to the decline in the deer population in the area. Additionally, given access to the ROW, deer came to the road-right-of way to graze, exacerbating the Wildlife-Vehicle Collision problem. It is possible that at-grade crossings can be effective on low traffic volume roads if used in conjunction with remotely sensed message board that report to the motorist when an animal is on the road. Currently (as of Summer 2007) there is a high technology crosswalk in place in Payson Arizona on State Highway 260. There are 11 wildlife underpasses in place along this roadway with 6 more planned. Fencing funnels wildlife to the passages but the fence ends in an area still prone to elk-vehicle collisions. At the end of the fence, there is monitoring equipment that senses if an animal enters the roadway; it triggers a driver message board and lights when an animal is on the road-right-of way. Drivers are expected to slow down to avoid a collision. Prior to the installation of these devices, the wildlife biologist heading the research on the project clocked 136 motorists driving over 100 mph in a two day period. Preliminary evidence suggests the system is working and motorists are slowing down considerably in this cross walk area.
Visit the following website for more information on Arizona crosswalks:
http://www.paysonroundup.com/section/frontpage_lead/story/26511
http://www.gatewaytosedona.com/article/id/1051/page/1

See Huisjer et al (2006) for a review of similar driver warning technologies. url: http://trb.org/news/blurb_detail.asp?id=6690/. See Gordon et al. 2004 for evaluation of driver warning systems in an area deer move across a roadway in Wyoming.

Underpasses

Wildlife underpasses are typically bridges or culverts that allow wildlife to pass underneath a road. In an effort to standardize the many types of underpasses, we have created the table below to categorize the types of underpasses based on size, type of structure, and function. The table below can be used to identify different size classes and provides preliminary specifics on configurations: more detail on dimensions is given in Step 2.3. Below the table are the references that we used to determine the breaks between the classes, and pictures of examples.

Types of Underpasses and Their General Dimensions, Functions, and Target Species

Examples of small underpasses

Desert tortoise using bridge underpass, Utah. Photo credit: A. McLuckie.

Turtle using culvert passage in Oregon. Photo credit: Port of Portland, OR.

Coyote using culvert underpass, Montana. Photo credit: K. Foresman.

Examples of Medium Underpasses

Tennessee Bear Culvert. Photo credit: K. Brown

Vermont Agency of Transportation and Vermont Fish and Wildlife personnel on a field trip to examine wildlife use tracks in a box culvert. Photo credit: C. Slesar.

Bobcat using medium size box culvert underpass, Florida.

Examples of Large Underpasses

Arizona bridge underpass for elk, which have been documented using it thousands of times. Photo credit: P. Cramer.

Corrugated steel (multi-plate) culvert for mule deer and elk in Utah. Only mule deer documented using passage in first two years post-construction. Photo credit: S. Rosa.

Quebec underpass, accommodating both stream and upland for terrestrial wildlife movement. Photo credit N. Desbiens

Florida extended bridge over creek, to accommodate black bear, otter, white-tailed deer, and other wildlife movement. At the time of photograph, deer and otter tracks were detected. Photo credit: P. Cramer.

Wildlife bridge underpass in Chino Hills, California. The initial wildlife culvert passages did not pass the targeted bobcat and coyote, so in an adaptive management strategy, Caltrans (California's Transportation Agency) removed the original box culvert and installed a bridge in an attempt to create an effective passage. Monitoring began in 2007. Photo credit: K. Sacilotto.

Wildlife crossings in San Diego region of California. Photo credit: B. April.

Idaho box culvert underpass. In first two years of monitoring, documented use by bear, moose, mule deer, bobcat and other species, but not used by elk. Photo credit: P. Cramer.

2.1.7 Other Mitigation Options

The main objective of this decision tool step is to help the user decide what type of wildlife passage options they may choose from. Wildlife passages are almost always placed in conjunction with other types of mitigation, such as fences and escape ramps. There are also adjustments and retrofits that can be made to existing passages under bridges, culverts, and fencing that can be made to make them more conducive to wildlife movement under the roadway. While a more detailed description of the specifics of these adjustments will be dealt with in Step 2.3, "Determine Configuration," we encourage the reader to follow the link to the Tijeras Canyon Safe Passages Coalition Summer 2007 Newsletter for a detailed example of how existing bridges, culverts and fences can be retrofitted to encourage wildlife movement under the road, in conjunction with new passages, Electrobraid fences and mats, and driver warning systems. If the user wishes to consider other options, such as devices that may further alert drivers to approaching wildlife, or alert wildlife to approaching motorists, we recommend a visit to the website: www.deercrash.com. In this site the authors review the traditional options to prevent deer-vehicle collisions and the efficacy of these methods. The Georgia Department of Transportation has released a report (2007) that explores the issue of deer-vehicle collisions, evaluates the effectiveness of wildlife warning reflectors for altering the behavior of white-tailed deer along roadways, and includes basic information on the sight and hearing capabilities of white-tailed deer. The report also examines ways to reduce the incidence of deer-vehicle collisions. This report may be downloaded from the url: http://trb.org/news/blurb_detail.asp?id=7947. There may be technological developments available in the future that would allow drivers of vehicles to become alerted to wildlife further down the road then what can be normally seen with normal vision. See: http://www.marlow.com/Applications/DSP/cadillac_night_vision_system.htm.

2.1.8 References

Ashley, P. I., A. Kosloski, and S. A. Petrie. 2007. Incidence of intentional vehicle-reptile collisions. Human Dimensions of Wildlife, 12:137-143. url: http://www.informaworld.com/smpp/content?content=10.1080/10871200701322423

Bellis, M. 2007. Evaluating the effectiveness of wildlife passage structures on the Bennington Bypass. In: Proceedings of the 2007 International Conference on Ecology and Transportation. Edited by C. L. Irwin, P.Garrett, and K.P. McDermott. Raleigh, NC: Center for Transportation and the Environment, North Carolina State University, North Carolina, in Press.

Bissonette, J.A., M. E. Lehnert, and J. W. Haefner. 2000a. Dead on the road: mitigative models to address deer highway mortality. Abstract P. 52. in Proceedings of the symposium: Wildlife and Highways: Seeking solutions to an ecological and socio-economic dilemma. 7th Ann. Meeting of the Wildlife Society, Nashville TN.

Bissonette, J. A., M. E. Lehnert, and M. Harrison. 2000b. Lanes of destruction: effectiveness of highway right-of-way escape structures for mule deer. Abstract P. 124. in Proceedings of the symposium: Wildlife and Highways: Seeking solutions to an ecological and socio-economic dilemma. 7th Ann. Meeting of the Wildlife Society, Nashville TN.

Clevenger, A.P, and N. Waltho. 2000. Factors influencing the effectiveness of wildlife underpasses in Banff National Park, Alberta, Canada. Conservation Biology. 14:47-56.

Clevenger, A. P., and N. Waltho. 2005. Performance indices to identify attributes of highway crossing structures facilitating movements of large mammals. Biological Conservation. 121:453-464.

Clevenger, A. P., B. Cruszcz, K. Gunson, and J. Wierzchowski. 2001. Highway mitigation fencing reduces wildlife-vehicle collisions. Wildlife Society Bulletin 29:646-653.

Clevenger, A. P., B. Chruszcz, and K. Gunson. 2001. Drainage culverts as habitat linkages and factors affecting passage by mammals. Journal of Applied Ecology 38:1340-1349.

Dodd, C.K., W.J. Barichivich, and L.L. Smith. 2004. Effectiveness of a barrier wall and culverts in reducing wildlife mortality on a heavily traveled highway in Florida. Biological Conservation: 118:619-631.

Foresman, K. 2003. Small mammal use of modified culverts on the Lolo south project of western Montana- an update. In 2003 Proceedings of the International Conference on Ecology and Transportation, edited by C. Leroy Irwin, Paul Garrett, and K.P. McDermott. Raleigh, NC: Center for Transportation and the Environment, North Carolina State University, 2003

Gordon, K. and S. Anderson. 2003. Evaluation of an underpass installed in U.S. Highway 30 at Nugget Canyon, Wyoming, for migrating mule deer. Wyoming Cooperative Fish and Wildlife Research Unit for the Wyoming Department of Transportation. FHWA-WY-3/01. Cheyenne, Wyoming.

Gordon, K.M., M.C. McKinstry, and S.H. Anderson. 2004. Motorist response to a deer-sensing warning system. Wildlife Society Bulletin 32: 565-573

Huijser, M. P., P. T. McGowen, W. Camel, A. Hardy, P. Wright, A. P. Clevenger, Alaska Department of Transportation and Public Facilities, Departments of Transportation of CA, IN, IA, KS, MD, MT, NV, NH, NY, ND, PA, WI, WY, and Federal Highway Administration. 2006. Animal vehicle crash mitigation using advanced technology Phase I: Review, design, and implementation. Final Report to Federal Highways. SPR-3(076). url: http://trb.org/news/blurb_detail.asp?id=6690.

Jackson, S. D. 1996. Underpass systems for amphibians. In G. L. Evink, P. Garrett, D.Seigler, and J. Berry (eds). Trends in Addressing Transportation Related Wildlife Mortality, Proceedings of the transportation related wildlife mortality seminar. State of Florida Department of Transportation, Tallahassee, FL. FL-ER-58-96.

Jackson, S. D., and T. F. Tyning. 1989. Effectiveness of drift fences and tunnels for moving spotted salamanders Ambystoma maculatum under roads. Pages 93-99 in T.E.S. Langton (ed). Amphibians and Roads, proceedings of the toad tunnel conference. ACO Polymer Products, Shefford, England.

Lehnert, J. E. and J. A. Bissonette. 1997. Effectiveness of highway crosswalk structures at reducing deer-vehicle collisions. Wildlife Society Bulletin, 25:809-818.

Lehnert, M. E., L. A. Romin, and J. A. Bissonette. 1996. Mule deer-highway mortality in northeastern Utah: causes, patterns, and a new mitigative technique. (9 pages) in G. L. Evink, P. Garrett, D. Zeigler, and J. Berry, eds., Trends in Addressing Transportation Related Wildlife Mortality. Proc. Transport. Related Wildlife Mortality Seminar, June 1996, Fla. Dept. Transport. Publ. No. FL-ER-58-96 Tallahassee Fla. (unpaginated).

Lehnert, M. E., J. A. Bissonette, and J. W. Haefner 1998. Deer (Cervidae) highway mortality: using models to tailor mitigative efforts. Proceedings of the International Union of Game Biologists 23rd Congress: Game Management and Land Use in Open Landscapes. Gibier Faune Savage, Game Wildl. Vol 15 (Hors serie Tome 3) 15:835-841.

McGuire, T. M., and J. G. Morrall. 2000. Strategic highway improvements to minimize environmental impacts within the Canadian Rocky Mountain National Parks. Canadian Journal of Civil Engineering x: 523-532.

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