Install barriers and crossing structures along roads/railways

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • Sixteen studies evaluated the effects of installing barriers and crossing structures along roads/railways on reptile populations. Five studies were in the USA, three were in each of Spain, Australia and Canada and one was in each of France and South Africa.



  • Survival (8 studies): Four of seven studies (including one randomized, controlled, before-and-after study and one review) in the USA, Australia, Canada and South Africa found that installing fencing and crossing structures did not reduce road mortalities of reptiles, and in one case the percentage of mortalities may have increased. Two studies found that areas with fencing and crossing structures had fewer road mortalities of turtles and overall reptiles. One study found that reptile road mortalities still occurred in in areas with roadside barrier walls and culverts. One replicated, before-and-after study in Canada found that following installation of tunnels and guide fencing, along with signs for motorists, there were fewer road mortalities of eastern massasauga rattlesnakes.


  • Use (12 studies): Six studies (including two replicated studies and one review) in Spain, France, the USA and Australia found that crossing structures with fencing that were not specifically designed for wildlife were used by lizards, snakes, tortoises, turtles and alligators and ophidians. One study also found that the addition of fencing around crossing structures did not affect the number of reptile crossings. Three studies (including one replicated and one before-and-after study and one review) in the USA and Spain found that wildlife crossing structures with fencing were used by gopher tortoises and 12 snake species, American alligators and lacertid lizards. One study also found that an American alligator did not use the wildlife crossing structure. Two before-and-after studies (including one controlled study) in Canada found mixed effects of installing roadside fencing and culverts on road use by turtles and snakes. One replicated study in Spain found that use of different crossing structures depended on species group. One replicated study in Australia found that reptiles used wildlife underpasses or culverts for only 1% of road crossings. One replicated, before-and-after study in Canada found that following installation of tunnels and guide fencing, along with signs for motorists, fewer eastern massasauga rattlesnakes were found crossing the road.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A study in 1991–1992 along a high-speed railway through agricultural land in Castilla La Mancha, Spain (Rodriguez et al. 1996) found that culverts and underpasses not specifically designed for wildlife were used as crossings under the railway by reptiles, but that the addition of fencing did not affect crossing rates. Lizards and snakes were recorded making 112 crossings, or 7 crossings/100 passage-days on average across 15 underpasses and two overpasses. Reptiles preferred culverts 2 m wide and used culverts or underpasses more frequently than overpasses. Fencing did not significantly affect relative crossing rates (data presented as statistical model result). Reptile crossing rates were lower in autumn–early spring and varied with habitat types. Fifteen dry culverts and passages (e.g. small roads and two overpasses, 13–64 m long, 1.2–6.0 m wide, 1.2–3.5 m high) along a 25 km section of high-speed railway, were monitored. The railway was fenced with 2 m high wire netting in July 1991–March 1992. Tracks in sand were monitored at each passage for 15–22 days/month between September 1991 and July 1992.

    Study and other actions tested
  2. A before-and-after study in 1993–1995 in forest and pasture in Florida, USA (Roof & Wooding 1996) found that after a fence and wildlife underpass was built, numbers of road-killed reptiles did not decrease, but tortoises and snakes used the crossing. Sample sizes were small and results were not statistically tested. In the year after a fenced wildlife underpass was installed, two box turtles Emydidae spp., one cooter Pseudemys concinna and one gopher tortoise Gopherus polyphemus were killed on the road, compared to one box turtle before installation. The underpass was used by two gopher tortoises and 12 snakes (species not identified). One alligator mississippiensis walked along the fence line but did not use the crossing. A wildlife underpass (14.3 m long, 7.3 m wide, 2.4 m high) and barrier fencing (3 m chain-link and barbed wire fence topped with three strands of barbed wire, both sides of the underpass, 1.7 km total length) was erected in 1994. Trails were cut into woodland and trees planted in pasture to guide wildlife to the underpass. Roadkill surveys were carried out three times/week pre-fencing (December 1993–November 1994) and post-fencing (December 1994–November 1995). In December 1994–December 1995, movements along the fence line and in the underpass were monitored by surveying tracks and a camera in the centre of the underpass.

    Study and other actions tested
  3. A study in 1989–1994 of roadside verges in Toulon, France (Guyot & Clobert 1997) found that some Hermann's tortoise Testudo hermannii used culverts or a road tunnel with fencing to cross a highway. Seven (three females, four males) of 70 individually-marked Hermann’s tortoises used a road tunnel or culverts to cross a highway. A highway was constructed between May 1989 and October 1990 through Hermann's tortoise habitat. Sheep wire fencing was erected to prevent tortoises from crossing the road and one road tunnel and two culverts were constructed to allow tortoise movements between the two sides of the highway. Resident tortoises (300 individuals) were temporarily relocated during construction and individually marked prior to release. In April–October 1993–1994, observers carried out visual searches for tortoises next to the highway, recording recaptures (70 relocated tortoises were recaptured) and individually marking new individuals.

    Study and other actions tested
  4. A replicated study in 2000 in nine roadside verges in coastal open woodland in New South Wales, Australia (Taylor & Goldingay 2003) found that wildlife underpasses (‘culverts’) with fencing were used by some lizards and snakes in a four-month period. Road underpasses were used 11 times by lizards, including three lace monitor Varanus varius crossings (the only species mentioned) and twice by snakes. Reptile use of the underpasses comprised 1% of all wildlife crossings (1,202 total crossings). Nine purpose-built wildlife underpasses were surveyed for wildlife crossings along a 1.4 km long section of road traversing coastal low-lying dunes. Both sides of the road were lined with a 180 cm high chain-mesh fence. Culverts were made from reinforced concrete with stone or silt floors. Reptiles were surveyed using sand strips across the middle of each culvert (1 m long, 2–3 cm deep). Sand was checked for tracks every second day for eight days in September 2000 and December 2000.

    Study and other actions tested
  5. A study in 2001–2002 along a highway in Florida, USA (Dodd et al. 2004) found that culverts, in areas with roadside barrier walls, were used by reptiles but road casualties still occurred. Seventeen reptile species were recorded using culverts. These included American alligators Alligator mississippiensis (in five culverts), four turtle species (four culverts), green anoles Anolis carolinensis (one culvert) and 11 snake species (seven culverts). During the same period, ≥22 reptile species were recorded dead on the road. The most frequent casualties were yellow ratsnake Elaphe obsolete (16 individuals), southern watersnake Nerodia fasciata (21), and DeKay’s brownsnake Storeria dekayi (54). Culverts reduced overall vertebrate road mortality, but separate reptile figures were not reported for before culverts were installed. Eight culverts (from 0.9 m diameter to 2.4 × 2.4 m cross-section, all 44 m long) were connected using prefabricated concrete barrier walls. Culverts were monitored from 14 March 2001 to 5 March 2002 using funnel traps, camera traps and sand track stations. Roadkills were monitored by walking the 3.2 km road over three consecutive days each week over the same period.

    Study and other actions tested
  6. A review of studies investigating culverts and road barriers in the USA (Jochimsen et al. 2004) found that some species used culverts and in some cases road casualties were reduced. Nine alligators Alligator mississippiensis used four fenced wildlife underpasses. Desert tortoise Gopherus agassizii and shovel-nosed snake Chionactis occipitali road casualties reduced after a barrier fence, 24 culverts and three bridges were installed and tortoises were recorded using the culverts. Although red-sided garter snakes Thamnophis sirtalis infernalis used tunnels, snake road casualties remained high during autumn migrations and only two timber rattlesnakes Crotalus horridus were recorded using a culvert in the two years following its construction. See original paper for details of each study.

    Study and other actions tested
  7. A before-and-after study in 2000–2003 along a highway in Florida, USA (Aresco 2005) found that turtle road mortality decreased following the installation of drift fencing leading to a culvert. Turtle mortality on a road, primarily Florida cooter Pseudemys floridana and yellow-bellied slider Trachemys scripta, decreased after drift fencing was added to a culvert (0.1 individuals/km/day) compared to beforehand (11.9 individuals/km/day). During the study >200 individual turtle tracks and >25 alligator tracks were observed in the culvert. There was no evidence of predation of turtles at the culvert. In 2000, vinyl fences (600–700 m long, 0.6 m high) were installed along each side of a four-lane causeway to divert animals towards a culvert (diameter: 3.5 m, length: 46.6 m, built in 1963–1965). The bottom fence edge was buried approximately 20 cm underground and fence ends curved away from the road after >80–100 m. The highway and culvert were monitored using visual searches two–four times/day before (February–April 2000) and after (April 2000–November 2003) the construction of the fence (1,367 total survey days).

    Study and other actions tested
  8. A before-and-after study in 2004–2007 in dry eucalypt woodland in Queensland, Australia (Bond & Jones 2008) found that after an exclusion fence and vegetated overpass (‘land-bridge’) were built, one snake was found dead on the road compared to two before. Before construction of the fencing and overpass, one brown tree snake Boiga irregularis and one eastern small-eyed snake Cryptophis nigrescens were found as road-kill in the area and after the underpasses were constructed one carpet python Morelia spilota was found. In 2004 an exclusion fence, made of rubberised metal mesh (2.5 m high, 5 cm underground) with a rubber sheet (0.5 m high) running around the base, was built along a road overpass (see original paper for details) to a forest boundary on both sides of the bridge. Road-killed animals were surveyed by vehicle in the early mornings twice weekly before construction started in April–July 2004 and weekly after construction was completed in February 2005 until June 2007. All species larger than a blue-tongued skink Tiliqua scincoides were recorded.

    Study and other actions tested
  9. A replicated study in 2002 of a highway in Zamora, Spain (Mata et al. 2005; same experimental set-up as Mata et al. 2008) found that underpasses and culverts, in areas with roadside barrier fencing, were used by reptiles. Lacertid lizards (Lacerta spp. and Podarcis spp.) and ophidians (snakes and legless lacertids) were recorded in circular culverts (lacertids: 1.49 crossings/day/structure, ophidians: 0.03), adapted culverts (1, 0.5), and open-span underpasses (0.07, 0.07). Lacertid lizards were also recorded in wildlife underpasses (0.86 crossings/day/structure). A total of 64 underpasses and culverts (30–150 m long) under a 72 km section of motorway were monitored. These included 33 circular drainage culverts (2 m diameter), 10 wildlife-adapted box culverts (2–3 m wide, 2 m high), 14 open-span underpasses (rural tracks/paths, 4–9 m wide, 4–6 m high) and seven wildlife underpasses (20 m wide, 5–7 m high). The motorway was barrier-fenced. Animal tracks were monitored over 10 days in June–September 2002 using marble dust (1 m wide across).

    Study and other actions tested
  10. A replicated study in 2001 along a highway in Zamora province, Spain (Mata et al. 2008; same experimental set-up as Mata et al. 2005) found that road underpasses and culverts, in areas with roadside barrier fencing, were used by lizards. Lacertid lizards (Lacerta spp. and Podarcis spp.) were recorded in circular culverts (lacertids: 0.36 crossings/day/structure) and open-span underpasses (0.14) but not adapted culverts or wildlife underpasses. Ophidians (snakes and legless lacertids) were not recorded in any underpasses. Thirty-three crossings were monitored. These comprised 14 circular drainage culverts (2 m diameter, 35–62 m long), seven wildlife-adapted box culverts (2–4 m wide, 2–3 m high, 36–45 m long), seven open-span underpasses (rural tracks/paths, 4–9 m wide, 4–6 m high, 32–72 m long) and five wildlife underpasses (14–20 m wide, 5–8 m high, 30–96 m long). The motorway had barrier fencing along its length. Animal tracks were recorded using marble dust (1 m wide cross) over 10 days in March–June 2001.

    Study and other actions tested
  11. A replicated, site comparison study in 2006–2007 on a highway through forest and agricultural land in North Carolina, USA (McCollister & van Manen 2010) found that fenced stretches of road with underpasses tended to have lower rates of reptile road mortality than those without. Results were not statistically tested. Reptile mortality on stretches of road with underpasses and fencing was 1 reptile/km (8 individuals) compared to 2 reptiles/km (26 individuals) on unfenced road with no underpasses. Some reptiles, e.g. snakes, were small enough to climb through the fencing (see original paper). A new four-lane highway was constructed in 2001–2005 with three underpasses (3 m high, 29–47 m wide). Each underpass had an 800 m fence either side of it, which ran parallel to the highway, then continued under the underpass and connected with fencing on the opposite side (3 m high chain-link fencing). A section of the highway (with underpasses and fencing 6,375 m; without: 10,873 m) was surveyed for wildlife casualties twice/week in July 2006–July 2007.

    Study and other actions tested
  12. A controlled, before-and-after study in 2012–2013 along a section of highway through wetlands, rocky outcrops and mixed forest in Ontario, Canada (Baxter-Gilbert 2015) found that installing fencing and culverts prevented an increase in road use by snakes, but may have increased the percentage of snakes and turtles that died on the road. The number of snakes and turtles (both dead and alive) discovered on roads stayed similar in areas with fencing and culverts (snakes: 0.6–0.7/day; turtles: 0.5/day), but without fencing and culverts snake numbers increased (before: 1.4/day; after: 2.4/day), but turtle numbers stayed the same (1.0–1.1/day). However, the percentage of dead reptiles may have increased with fencing and culverts (before: 68% of turtles, 68% of snakes; after: 86% of turtles, 90% of snakes), but stayed similar in the area without (before: 86% of turtles, 76% of snakes; after: 88% of turtles, 88% of snakes), but this result was not tested statistically. In 2012, three crossing structures (culverts) were installed along a 13 km section of highway and connected with fencing (plastic sheeting and a chain-link fence). A similar 13 km section of highway with no fencing or culverts was also selected. In May–August in 2012 (before installation) and 2013 (after installation), both sections of highway were surveyed by car (13 km section; 3 surveys/day) or by foot (2 km section; 1 survey/day) to count the number of live and dead reptiles on the road.

    Study and other actions tested
  13. A replicated study in 2012–2013 in southern metropolitan Perth, Western Australia, Australia (Chambers & Bencini 2015) found that western bobtail lizards Tiliqua rugosa used underpasses of all sizes and shapes under fenced roads, and five other reptile species were observed using underpasses. Bobtail lizards used all 10 underpasses with 3–148 total crossings/underpass made by 1–8 individual lizards/underpass. Bobtail lizard use of underpasses was not related to length (23–88 m), cross-sectional area (0.3–1.4 m2), presence of logs or sticks, surrounding vegetation cover (0–50%), presence of predators, or time since construction of the underpass (2–19 years). Other reptile species seen using the underpasses included Gould’s sand monitor Vaanus gouldii, western bluetongue Tiliqua occipitalis, southern heath monitor Varanus rosenbergi, black-headed monitor Varanus tristis and dugite Pseudonaja affinis. Road crossings were monitored through 10 underpasses (round: 0.6–0.9 m diameter or square culverts: 0.6–1.2 m wide, 0.5–1.2 m high) from May 2012 to May 2013. All roads were fenced (600–1,800 mm high, buried 300 mm deep). Bobtail lizards were trapped and individually marked using PIT tags in the vicinity of each underpass for four consecutive nights. Underpasses were equipped with PIT tag reader antennas and infrared motion-sensor cameras installed to record animals on either side of the culvert.

    Study and other actions tested
  14. A replicated, before-and-after study in 2002–2014 on three roads in Ontario, Canada (Colley et al. 2017) found that eastern massasauga rattlesnake Sistrurus catenatus mortality was reduced after tunnels and guide fencing, along with signs for motorists, were installed. The number of rattlesnakes found dead or alive on roads decreased after tunnel, fence and sign installation (dead: 6, alive: 14) compared to before installation (dead: 41, alive: 68) and during installation (dead: 15, alive: 37). Fourteen of 68 individually-marked (‘PIT tagged’) rattlesnakes were recorded using tunnels. Rattlesnakes and garter snakes Thamnophis sirtalis showed a similar willingness to enter tunnels (rattlesnakes: 18 of 19; garter snakes: 15 of 16) and complete crossings to the other side (rattlesnakes: 7 of 19; garter snakes: 5 of 16). In 2007–2013, three mesh barrier fences 600–900 m long were installed on one or two sides of the road). In 2010–2011, a grate-top tunnel (8.5 m long x 1.2 m wide x 0.5–0.6 m deep) was installed in two of the fenced sections. Four signs to encourage motorists to slow down for snakes were installed near known snake road crossing locations. Rattlesnakes were surveyed on the road in May–October before (2005–2007) and during installation (2008–2012) by car and after installation (2013–2014) by bicycle. Tunnel use was monitored with camera traps and automated PIT tag readers. In 2014, rattlesnakes and garter snakes were caught opportunistically and placed at tunnel entrances to test willingness to enter and complete tunnel crossings.

    Study and other actions tested
  15. A randomized, controlled, before-and-after study in 2015 along a grassy verge in Limpopo Province, South Africa (Collinson et al. 2017) found that adding trenches or barriers to direct reptiles to concrete road underpasses (‘culverts’) did not reduce reptile roadkill numbers. Overall numbers of small terrestrial vertebrates killed were statistically similar after trenches or barriers were erected (before: 0.33 roadkill/day/km vs after: 0.04 roadkill/day/km). After trenches or barriers were erected 0–1 reptiles were killed on the road compared to 1–2 individuals before they were put in place. Based on areas of high roadkill, pre-existing concrete culverts on a 12.3 km long road stretch were selected and randomly chosen to be adapted by the addition of a barrier (70 cm high, 30 cm below ground, 3 culverts), or a trench (30 cm deep, 3 culverts), or no changes were made (3 culverts). Trenches and barriers were built parallel to the road (approximately 2 m from the edge), 200 m long either side of the culvert (400 m total length). Roadkill surveys were carried out by vehicle for 20 consecutive days before trenches and barriers were built in January 2015 and afterwards in February 2015 (20 consecutive days).

    Study and other actions tested
  16. A before-and-after study in 2003–2014 in a wetland complex in Ontario, Canada (Markle et al. 2017) found that adding roadside fencing and culverts reduced turtle and snake abundances on a causeway, although only along completely fenced sections of road, and use of culverts by individuals was low. In areas that were fully fenced, the number of turtle or snakes found on the causeway was lower after fencing than before (turtles: after fencing: 2, before fencing: 10; snakes: after: 3, before: 7), but remained similar in partially fenced areas (turtles: 3; snakes: 3–4) and areas with no fencing (turtles: 1–2; snakes: 2). Two of 68 Blanding’s turtles Emydoidea blandingii and none of 30 spotted turtles Clemmys guttata used one of seven culverts. One of 30 radio-tracked Blanding’s turtle used a culvert once. Reptiles were surveyed in April–October on a causeway (3.6 km long) across a marsh for five years (2003–2007: 22 surveys/month, 154 total surveys) before post-and-mesh-net fencing was installed in 2008–2009 and for five years afterwards (in 2010–2014: 40 surveys/month, 284 total surveys). After exclusion fencing was built, road sections were classified as: fully fenced, partially fenced, or unfenced. In 2012–2014, seven culverts were added to the causeway. In 2014–2015, culvert use was monitored by cameras, an automated PIT-tag checker at culvert entrances (68 Blanding’s and 30 spotted turtles were PIT-tagged) and radio-tracking turtles (30 additional Blanding’s turtles were radio-tracked once a week during active seasons).

    Study and other actions tested
Please cite as:

Sainsbury K.A., Morgan W.H., Watson M., Rotem G., Bouskila A., Smith R.K. & Sutherland W.J. (2021) Reptile Conservation: Global Evidence for the Effects of Interventions for reptiles. Conservation Evidence Series Synopsis. University of Cambridge, Cambridge, UK.

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