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1 December 2012 An evaluation of a mitigation strategy for deer-vehicle collisions
John A. Bissonette, Silvia Rosa
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High mule deer Odocoileus hemionus mortality in southwestern Utah led to the establishment of a mitigation strategy with two major objectives: 1) reduction of wildlife-vehicle collisions and 2) restoration of landscape connectivity to facilitate wildlife movement across the roaded landscape. During our study, we assessed the effectiveness of the mitigation measures in reducing mule deer mortality in the following ways: 1) we compared the number of deer-vehicle collisions in the newly fenced area with a control area without fencing; 2) we analyzed the ‘end-of-the-fence’ problem, defined here as increased mortality of mule deer at the ends of the 2.4-m high exclusion fences; and 3) we evaluated the frequency of animal crossings of the new underpasses using remotely-sensed cameras and compared them with crossing frequency rates for a 20-year-old control underpass. We compared six years of pre-construction mortality (during 1998-2003) with two years of post-construction data on mortality (during 2005-2006) and found a 98.5% decline in deer mortalities in the treatment (i.e. fenced, jump-outs and underpasses) vs a 2.9% decline in the control (i.e. no fences, no jump-outs and no underpasses). We detected no end-of-the-fence problems related to deer mortality. Migratory movements during fall and spring were clearly reflected in the use of underpass. Overall results demonstrated that the mitigation strategy was effective and reduced the number of deer-vehicle accidents, while allowing wildlife movement across the landscape.

The increase in animal-vehicle collisions worldwide is a reflection of the increased anthropogenic transformation of the landscape. Current estimates of the total landscape transformed by human influences (e.g. forest removal, agricultural activities including crop planting and livestock grazing) range from 39 to 50% of the total earth surface (Vitousek et al. 1997). When Riitters & Wickham (2003) measured the proximity of different land-cover types in the U.S. to the proximity of a road using nine distance classes, they found that the proportion of land area within a defined distance to a road increased rapidly with distance. Of all land area in the U.S., 50% was within 382 m of a road; only 18% was > 1,000 m of a road. Clearly, as road density increases as a function of human development, so does mean animal proximity to roads. The U.S. has a mean road density of 0.75 km/km2 with concomitant habitat loss, animal mortality, isolation and barrier effects (Jaeger & Fahrig 2004, Jaeger et al. 2005, Row et al. 2007).

As road networks have expanded, so have traffic volumes, which have been linked to deer mortality (Gunson et al. 2006). In the U.S., traffic volumes in 1998 totaled 2,625,367 × 106 vehicle miles; in 2007 traffic volume had increased to 3,003,218 × 106, ∼ a 14.4 % increase (Available at: For Utah, annual vehicle-miles traveled (VMT) in 2007 totaled 26.8 billion miles (Available at:,T:,530), an increase of ∼ 26.4 % from the 1998 VMT of 21.2 billion miles.

One of the most evident effects of the increased traffic volume on wildlife is the increased number of wildlife-vehicle collisions (Bissonette & Cramer 2008, McCollister & van Manen 2010). Even though Skölving (1987), Berthoud (1987) and Seiler (2005) suggested that high traffic volumes tended to cause road avoidance by animals; nevertheless, nation-wide estimates of 1.1 million deer-vehicle collisions (DVCs) during the period between 1 July 2008 and 30 June 2009, or ∼ 21,150/week (State Farm Insurance Company® 2009), reveal the growing importance of the problem in the U.S. When seven years of data were recalculated in 3-year time steps to smoothen extraordinary high or low years of DVCs, 2006-2009 showed a 15.3% increase over the previous three years. The comparable figure for Utah is 25%. State Farm® estimated that the likelihood of any vehicle colliding with a deer in 2010 was 1:208 for the U.S. and 1:404 for Utah. In the Intermountain West, wildlife-vehicle collisions primarily involve ungulates in general and mule deer Odocoileus hemionus in particular. In Utah, mule deer numbers have declined generally over the past several decades with current estimates of < 300,000 animals, from an estimated 500,000 deer in the 1950s (Utah Department Wildlife Resources 2009).

In 2003, to increase driver's safety, reduce deer mortality and provide deer access to their traditional seasonal ranges across I-15, three agencies (Utah Division of Wildlife Resources (UDWR), Utah Department of Transportation (UDOT) and the Bureau of Land Management (BLM)) jointly created a mitigation strategy that included three integrated actions: 1) construction of two wildlife underpasses, 2) construction of exclusion fencing and 3) installation of 1-way earthen escape ramps. The underpasses were constructed to allow below-grade road crossing, thereby reducing the putative barrier effect created by wildlife exclusion fencing and the road (Foster & Humphrey 1995, Bruinderink & Hazebroek 1996, Jaeger & Fahrig 2004). Exclusion fencing is seldom, if ever, 100% effective, even with continued maintenance (Putman 1997), so earthen 1-way escape ramps were constructed being spaced ½ mile apart to allow deer that accessed the fenced right-of-way (ROW) an escape route. The overall objectives of the mitigation were to improve driver safety, reduce the number of DVCs and to restore landscape connectivity for migrating deer.

For the mitigation strategy to be considered effective and because formally established criteria were unavailable, we established three a priori criteria. First, DVCs on the treatment section needed to be reduced by ≥ 70%. We chose this threshold based on studies of mule deer in northern Utah (Lehnert 1996, Lehnert et al. 2008) and reductions in mortality observed in successful mitigations strategies elsewhere (McDonald 1991, Clevenger et al. 2001). Second, there should not be an increase in DVCs at the ends of the exclusion fencing (i.e. the so-called ‘end-of-the-fence’ problem; Bellis & Graves 1971, Clevenger et al. 2001). Third, because of habituation, there should be an increase in underpass use by deer, and use should increase with time (Ward 1982).

We conducted a study using six years of pre-construction data (1998-2003) and three years of post-construction data (2004-2006). We examined if fences and escape-ramps jointly reduced deer mortality on the road, and if underpasses were used by mule deer during seasonal migrations