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Providing evidence to improve practice

Action: Install one-way gates or other structures to allow wildlife to leave roadways Terrestrial Mammal Conservation

Key messages

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  • Seven studies evaluated the effects on mammals of installing one-way gates or other structures to allow wildlife to leave roadways. All seven studies were in the USA.

COMMUNITY RESPONSE (5 STUDIES)

  • Survival (5 studies): Two before-and-after studies (one replicated), in the USA, found that barrier fencing with one-way gates reduced deer-vehicle collisions. One of two studies (one before-and-after and one replicated, controlled), in the USA, found that barrier fencing with escape gates along roads with one or more underpasses reduced moose-vehicle collisions, whilst the other found no reduction in total mammal road casualty rates. A replicated, controlled, before-and-after study in USA found that earth escape ramps reduced mammal road mortalities.

POPULATION RESPONSE (0 STUDIES)

BEHAVIOUR (4 STUDIES)

  • Use (4 studies): One of two studies (one replicated) in the USA, found that one-way gates allowed mule deer to escape when trapped along highways with barrier fencing, whilst the other found that a small proportion used one-way gates. A replicated, controlled, before-and-after study in the USA found that earth escape ramps were used more often than were one-way escape gates to enable deer to escape highways with barrier fencing. A replicated, controlled study in the USA found that barrier fencing with escape gates and underpasses facilitated road crossings by a range of mammals.

Supporting evidence from individual studies

1 

A replicated study in 1970–1972 in Colorado, USA (Reed et al. 1974) found that one-way gates allowed mule deer Odocoileush emionush emionus to escape when trapped along highways with barrier fencing. A total of 558 passages were recorded through eight gates, with 96% in the one-way direction designed. Use of each gate ranged from seven to 335 passages. Track counts indicated that the gates enabled approximately 223 deer to escape the highway. There were also 3,293 tracks counted of deer approaching gates heading towards the highway but not passing through. During 31 trails, three types of one-way gate were tested (two at a time) along a fence between a field with a mule deer and one with its food. The location and direction of each gate was changed frequently. Eight gates, of the most effective design, were installed in 2.4-m-high barrier fencing along a 1.5-mile section of highway. Passages were monitored using track counts and mechanical counters. Gates along the highway were checked daily during migrations in 1970–1972.

2 

A before-and-after study in the 1970s along two highways in California, USA (Reed et al. 1982) found that barrier fencing incorporating one-way gates reduced deer-vehicle collisions by 68–87%. Fewer deer Odocoileus spp. road mortalities were recorded after construction of the six fence sections (average 2/km/year) than before (average 11/km/year). Six different lengths (1.9–7.7 km) of 2.4-m fencing were installed along Interstate 70 and Colorado Highway 82. Five of the fences were only on one side of the road, the other was on both sides and connected to an underpass. Four of the fences had one-way gates to allow deer to escape from the highway. Deer carcasses found along the road were counted in each fenced area before and after installation. Cost-benefit analysis was also undertaken using pre-fence mortality (dead deer) and fence effectiveness and estimates of cost of vehicle repair, deer value, discount rate, cost of fence and cost of fence maintenance (see original article for results).

3 

A replicated, before-and-after study in 1977–1979 along two highways in Minnesota, USA (Ludwig & Bremicker 1983) found that barrier fencing with one-way gates decreased deer-vehicle collisions. Along two fenced road sections, 1.3 and 8 deer/year were killed compared to an estimated 20/year in the pre-fence period. One fence was installed in a ditch with 1 m of water, meaning 30% of gates could not be used to escape the highway. Overall, 69% of 51 passages through gates were in the correct direction, i.e. from the highway to outside the fenced corridor. Two sections of 2.4-m-high fence with one-way gates along new highways were monitored for 18 months. Fences were 4 and 5 km long with nine and 10 pairs of gates (30 m apart), respectively. Deer were monitored crossing through gates by using baler counters and track beds. Deer-vehicle collisions were monitored for one year before (along old adjacent highway) and 18 months after installation. Cost-benefit analysis was also carried out (see the original article for further details).

4 

A before-and-after study in 1977–1990 in Alaska, USA (McDonald 1991) found that barrier fencing with one-way gates, along with an underpass and road lighting, reduced vehicle collisions with moose Alces alces. Effects of fencing, gates, lighting and the underpass could not be separated. There were fewer moose-vehicle collisions after installation of fencing with one-way gates, an underpass and lighting (0.7/year) than before (17/year). There was no significant difference in the distribution of moose in relation to the highway between after and before fence installation. A total of 17 moose were observed using one-way gates and tracks suggested gates were used frequently. However, this meant that moose were regularly getting onto the highway. The first gates installed stayed open if swung all the way open and gates got stuck open below 0°C, because of the lubricant used. In October 1987, road lighting was installed along 11.5 km of the highway. Fencing and 30 one-way gates were installed along 5.5 km of this section and an underpass was created. Moose-vehicle collisions were monitored before (1977–1987) and after (1987–1990) installation. One-way gates were monitored using track counts in snow.

5 

A study in 1994–1995 along two highways through grassland and shrubland in Utah, USA (Lehnert & Bissonette 1997) found that one-way gates were used by some mule deer Odocoileus hemionus to escape a highway, but most did not cross through them. From 243 instances in which deer approached gates from the highway, 40 deer (16%) used gates to leave the highway. None of 128 deer that approached from the side away from the highway passed through gates. In September 1994, five and four crossing points were installed along a two- and a four-lane highway respectively. Fencing, 2.3 m high, directed deer to crossing points. Warning signs alerted approaching motorists to crossing points. Four one-way gates were installed at each crossing to allow deer trapped along the road to escape. One-way gate specifications were not detailed in the paper. Earthen track beds at 12 randomly selected one-way gates were checked at least once each week from September 1994 to November 1995 (except January–March 1995).

6 

A replicated, controlled, before-and-after study in 1997–1999 along two highways in Utah, USA (Bissonette & Hammer 2000) found that earth escape ramps reduced road mortalities and were used more often than one-way escape gates to enable deer to escape highways with 2.4-m-high barrier fencing. Road mortalities decreased more after ramp installations at two sites (after: 4.8 and 2.0 killed/km; before: 6.7 and 4.6 killed/km) than at a control site during this time (after: 4.0 killed/km; before: 5.2 killed/km). At one site, 188 successful ramp crossings were recorded. At the other, 192 were recorded. Combined values from both sites showed ramps were used 8–11 times more often than were one-way gates. Nine earth ramps (1.5-m drop-off) were installed along 2.4 km of highway in 1997 and seven along 2.4 km of another highway in 1998. Ten and eight one-way gates respectively were installed previously at these sites (installation date not stated). Animal movements across ramps and through gates were monitored from May–July until October in 1998 and 1999 using track plots. Road mortality and monthly spotlight counts of deer were carried out before and after construction of ramps along both sections, and along an 8-km control section (1-m fencing, no mitigation measures) in 1997–1999. Cost-benefit analysis was also carried out (see original article for results).

7 

A replicated, controlled study in 2000–2007 along a highway in North Carolina, USA (McCollister & van Manen 2010) found that barrier fencing with escape gates and underpasses facilitated road crossings by a range of mammals but did not reduce road casualties. A similar rate of mammal road casualties was recorded over one year on road sections with fencing, escape gates and underpasses (5.0/km) as on sections without (5.1/km). A four-lane highway was constructed with three underpasses. Barrier fencing, 3 m high, was installed ≥800 m along the highway from each underpass. Gates allowed trapped animals to escape the highway. Road deaths were recorded along 6 km of road with fencing and underpasses and 11 km without, twice/week, from July 2006–July 2007.

Referenced papers

Please cite as:

Littlewood, N.A., Rocha, R., Smith, R.K., Martin, P.A., Lockhart, S.L., Schoonover, R.F., Wilman, E., Bladon, A.J., Sainsbury, K.A., Pimm S. and Sutherland, W.J. (2020) Terrestrial Mammal Conservation: Global Evidence for the Effects of Interventions for terrestrial mammals excluding bats and primates. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.