Install tunnels/culverts/underpasses under roads/railways

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

Study locations

Key messages



  • Survival (3 studies): Two site comparison studies (including one before-and-after study) in Australia and South Africa found a similar number of reptile road mortalities with or without culverts or wildlife underpasses. One replicated study in Spain found that the number of underpasses in an area did not affect the number of reptile road mortalities.


  • Use (12 studies): Six studies (including four replicated studies and one replicated, before-and-after study) and one review in Spain, Australia, the USA and Australia, Europe and North America found that crossing structures, including tunnels, culverts, underpasses, pipes and trenches under roads and railways were used by reptiles, lizards, snakes and/or tortoises. One review in Australia, Europe and North America also found that wildlife underpasses were used by reptiles in only one of 13 studies. Three of four replicated studies (including one before-and-after study) in the USA and Canada found that desert tortoises, painted and snapping turtles and rattlesnakes and garter snakes showed a willingness to enter some, or all types of tunnel. The other study found that only 9% of painted turtles entered a culvert during a choice experiment. One site comparison study in Australia found that the area under an overpass was used by five reptile species.

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 replicated study in 1992 in an outdoor facility in Nevada, USA (Ruby et al. 1994) found that over half of desert tortoises Gopherus agassizii entered tunnels of a suitable size during trials, of which around half went all the way through tunnels. After 30 minutes, 12 of 16 tortoises had entered tunnels of a suitable width for their size and seven of the 12 tortoises escaped through or moved to the end of the tunnel. Two tortoises stopped in the tunnel and three tortoises entered the tunnel and returned to the pen. Four tortoises investigated tunnel openings but did not enter. In September 1992, sixteen tortoises were placed individually in the middle of a walled pen (4.6 x 4.6 m) with five tunnels of varying diameters and lengths located around the pen edge providing a choice of exit points (narrow: 10 cm wide x 150 cm long and 10 cm wide x 90 cm long; medium: 19 cm wide x 120 cm long; wide: 29 cm wide x 280 cm long and 29 cm wide x 136 cm long). Tortoise behaviour was observed for 30 minutes and recorded.

    Study and other actions tested
  2. A replicated study in 1994 of 17 culverts under roads and railways in Madrid province, Spain (Yanes et al. 1995) found that culverts were used by snakes and lizards. Lizards (0.23 tracks/day) and snakes (0.02 tracks/day) were detected. Snakes and lizards were detected in culverts more often in the summer (0.88 tracks/day) compared to spring (0.04 tracks/day), autumn (0.04 tracks/day) or winter (0 tracks/day). Culvert use by snakes and lizards declined when detritus pits were recorded near the culvert (see original paper for details). Five culverts were monitored under railways, two under a motorway and 10 under local roads. Structural, vegetation and traffic variables were recorded at each culvert. Use was monitored using marble (rock) dust over culvert floors to record tracks. Sampling was undertaken in 1994, over four days each in spring, summer, autumn and winter. Sampling extended to eight days at four culverts when deer were nearby.

    Study and other actions tested
  3. A replicated study in 1993–1994 in four motorway roadside verges in Catalonia, Spain (Rosell et al. 1997) found that road underpasses (‘culverts’) were used by reptiles and that reptiles were recorded more often in circular than rectangular underpasses. Results were not statistically tested. Reptiles used four of 17 rectangle-shaped underpasses and 23 of 39 circle-shaped underpasses. The authors reported that reptiles were only recorded crossing shorter-length underpasses and were more likely to be recorded when there was natural substrate on the floor of the underpass and the opening of the structure was at ground level. In November 1993–September 1994, fifty-six underpasses (including drainage channels) along four 10 km-long stretches of motorway were surveyed for use by reptiles. Thirty-nine underpasses were circular in cross section (1–3 m diameter) and 17 were rectangular (4–12 m wide). Each underpass was monitored four times for four days/season (16 days in total/underpass) using a 50 cm long strip of powder substrate across each underpass and infra-red and photographic cameras.

    Study and other actions tested
  4. A replicated study in 1996–1997 along three roadside verges in New South Wales, Australia (Norman et al. 1998) found that road underpasses were used by three lizard species and one snake species. Over nine months, two of three underpasses were used by eastern water dragons Physignathus lesueurii (3 photographs, tracks observed in one underpass) and eastern water skinks Eulamprus quoyii (3 photographs, tracks observed in two underpasses). One of the underpasses was used by lace monitor Varanus varius (8 photographs, tracks and scats observed) and diamond python Morelia spilota ssp. spilota (one photograph). In mid-August 1996 to mid-June 1997, a camera with an infrared trigger was set in three different underpasses of different sizes and design on a highway. A 1 m wide sand tray was also placed in each underpass. The authors note that animals that were small enough to avoid triggering the camera may not have been consistently recorded.

    Study and other actions tested
  5. A site comparison study in 2002–2003 in eucalypt forest in Victoria, Australia (Abson & Lawrence 2003) found that 4–5 years after a flyover was built to enable wildlife to cross underneath a dual carriageway, some reptiles were present underneath the flyover as well as in adjacent forest. Four–five years after a road flyover was built, five species of reptile were counted underneath the flyover and five species of reptile were counted in forest adjacent to the flyover. In 1998 a dual carriageway road flyover was built across a tract of forest. Mature eucalypts and middle and understorey vegetation were kept during construction and native plant species were planted to maintain a similar vegetation structure to adjacent forest. Reptile use of the flyover was monitored monthly in July 2002–June 2003 using 14 different methods including pitfall trapping, sand trays and visual surveys for roadkill (see original paper for details).

    Study and other actions tested
  6. A before-and-after, site comparison study in 2000–2003 in forest in Queensland, Australia (Goosem et al. 2005) found that installing road underpasses did not reduce numbers of reptiles killed on a highway (although reptile numbers recorded were very low). In the two years after three road underpasses were installed under a highway, two reptiles were counted dead on the road, compared to one reptile in the year before underpass construction. As a comparison, numbers of roadkill reptiles counted on a nearby section of road without an underpass were higher (1 year after installation: 22 reptiles killed; 2 years after: 26 killed). In 2001, a high-altitude highway through rainforest was widened and upgraded to include three concrete wildlife underpasses (3.4 m high, 3.7 m wide). Underpasses incorporated ground cover to simulate the forest floor and arboreal structures (see paper for details). For 12 months prior to underpass construction, two 0.5 km long road transects were surveyed weekly for roadkill by walking either side of the highway. After underpass construction, reptile roadkill was monitored by walking 0.5 km long road transects. Two similar transects were walked on a highway without underpasses 5 km north of the upgraded highway.

    Study and other actions tested
  7. A replicated study in 2004–2005 along seven roads in Virginia, USA (Donaldson 2007) found that underpasses (including areas under bridges) were used by black rat snakes Pantherophis obsoletus. Black rat snakes were observed using at least one of the underpasses (data not provided). In June 2004–May 2005, seven underpasses (including the area under two bridges) were monitored using a camera at each entrance and exit. Photographs were downloaded once a week. Most of the underpasses were designed for water drainage.

    Study and other actions tested
  8. A replicated, before-and-after study in 2004–2007 in dry eucalypt woodland in Queensland, Australia (Bond & Jones 2008) found that two road underpasses were used by reptiles from six months after construction was finished. Results were not statistically tested. Six to 12 months after construction of two underpasses, 0.6–1.3 lizards/day and 18–30 months after construction, 0.1–1.0 lizards/day were recorded using the underpasses. The authors reported that most of the reptiles recorded were medium-sized lizards. After construction, one snake Morelia spilota was found dead on the road, compared to two snakes (one Boiga irregularis and Cryptophis nigrescens) beforehand. In 2004, two underpasses were constructed (2.4 m high, 2.5 m wide, 48 m long) containing: a lower level (0.9 m wide), a raised level with rocks (1.6 m wide, 0.4 m above ground), a half log railing, and a wooden shelf (0.25 m wide, 1.2 m above ground) running the length of the underpass. Sand strips (1–2 cm deep, 1 m long, 2.5 m wide) were placed 1–2 m inside the underpasses at both ends and on the shelves (0.5 cm deep, 0.5 m long, 0.25 m wide). Sand was checked for tracks twice weekly in August 2005–February 2006 and monthly in June 2006–June 2007. Animals killed on the road 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–June 2007. All species larger than a blue-tongued skink Tiliqua scincoides were recorded.

    Study and other actions tested
  9. A replicated study in 2005–2006 of tunnels in a Wildlife Management Area in New York, USA (Woltz et al. 2008) found that painted Chrysemis picta and snapping turtles Chelydra serpentina showed some preferences for particular tunnel widths and lengths, but not for different substrates or light levels. Both turtle selected mid-sized diameter tunnels (0.5 m: selected by 39–44% of individuals, 0.6 m: 31–39%) more often than narrower (0.3 m diameter: 6–17%),  or wider tunnels (0.8 m: 6–19%). Painted turtles avoided the longest tunnels (3 m long: selected by 10–30% of individuals, 6.1 m: 45%, 9.1 m: 15%), whereas snapping turtles did not show any significant preference for tunnel length (3 m: 20–37%, 6.1 m: 27%, 9.1 m: 17%). Neither turtle showed preferences for substrate type (concrete: 19–37%, gravel: 24–29%, soil: 20–26%, PVC: 20–26%) or light permeability (0% permeability: 26–31%, 0.6%: 14–23%, 1.3%: 15–24%, 5%: 31–36%). Snapping turtles were more likely to not choose any of the tunnels (56%) than painted turtles (16%). Choice arenas had four different PVC culverts radiating out, which painted (74 individuals) and snapping turtles (62) could select to exit through. Separate arenas were constructed to test tunnel diameter (0.3, 0.5, 0.6 or 0.8 m), length (3, 6.1 or 9.1 m), substrate (concrete, soil, gravel or bare PVC) and light (overhead punctures of 0, 0.6, 1.3 or 4% of surface). Turtles were tested individually, once/arena. Trials lasted 15 minutes, after 5 minutes acclimatization, in June–August 2005–2006.

    Study and other actions tested
  10. A review in 2010 of studies monitoring 329 road crossing structures in Australia, Europe and North America (Taylor & Goldingay 2010) found that reptiles used crossing structures in 21 of 37 studies. From a total of 37 studies, reptiles used pipes in four of five studies, drainage culverts in nine studies (total number of studies unclear), adapted culverts in four of six studies and bridge underpasses in three of seven studies. Reptiles used a wildlife underpass in one of 13 studies, and in one study they were seen, but did not use the structure. A database (Web of Science) was searched for peer-reviewed, English language studies published in 1998–2008, using a range of keywords relating to roads and wildlife (see paper for details), and reference lists of any papers obtained were also checked.

    Study and other actions tested
  11. A replicated study in 2013 along a section of highway through wetlands, rocky outcrops and mixed forest in Ontario, Canada (Baxter-Gilbert et al. 2015) found that few painted turtles Chrysemys picta entered a culvert under the highway. In a choice experiment, only 9% (5 of 54) of painted turtles Chrysemys picta entered the culvert, whereas 22% (12 of 54) moved away from the culvert and 69% (37 of 54) remained at the entrance. Authors reported that the percentage of turtles entering the culvert was lower than that recorded in a previous arena study away from the highway (47% crossed). In 2013, adult painted turtles (30 females; 24 males) were caught in the wild and brought to a culvert that had been constructed under a highway (2.5 km from capture location). The culvert (and highway) was located between the individuals and their home range. Turtles were allowed to acclimate for 10 minutes in an open box near the entrance to the culvert. The box was then removed and movements were monitored to see if they used the culvert, moved away from it, or did not move.

    Study and other actions tested
  12. A replicated, before-and-after study in 2002–2014 on three roads in Ontario, Canada (Colley et al. 2017) found that two snake species showed similar willingness to enter tunnels. Eastern massasauga rattlesnakes Sistrurus catenatus and garter snakes Thamnophis sirtalis showed a similar willingness to enter tunnels (rattlesnakes: 18 of 19 entered; garter snakes: 15 of 16) and to complete crossings to the other side (rattlesnakes: 7 of 19; garter snakes: 5 of 16). In 2010–2011, two grate-top tunnels (8.5 m long x 1.2 m wide x 0.5–0.6 m deep) were installed along a road. 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
  13. A site comparison study in 2015 along a grassy verge in Limpopo Province, South Africa (Collinson et al. 2017) found that incidences of reptile road casualties were the same whether or not there were concrete underpasses (‘culverts’) installed. Results were not statistically tested. When there were culverts, numbers of reptiles killed on the road were the same as when there were not (26 individuals found in both circumstances). A road with 17 culverts was selected. Surveys of reptiles killed on the road were carried out at sunrise in a vehicle along a 12.3 km long stretch of road for 20 consecutive days each in January and February 2015.

    Study and other actions tested
  14. A replicated study in 2009–2010 in mixed oak woodland and shrubland in west Andalusia, Spain (Delgado et al. 2018) found that higher numbers and density of underpasses did not reduce numbers of reptile road casualties. Data were reported as statistical model outputs. Spatial distribution of underpasses was not associated with patterns of reptile road casualties, which were found on average 100 m from the nearest underpass. In total 55 reptile carcasses were found (0.1–2.6 reptiles/km). Four roads were surveyed (53 km in total) for reptile carcasses on foot by 1–4 observers during October 2009–January 2010 and April–July 2010 (30 days surveying in total). Each road was surveyed twice. Underpasses were existing culverts or other road drainage tunnels.

    Study and other actions tested
  15. A study in 2016 on an inactive railway line in coastal strand habitat in east-central Florida, USA (Rautsaw et al. 2018) found that gopher tortoises Gopherus polyphemus used trenches dug under the rails to move from one side of the railway line to the other. Results were not statistically tested. In total, 68 of 96 tortoise photographs showed animals using the trenches (0.4 tortoise sightings/day). Trenches began to be used within four days of being dug. Two trenches were dug 700 m apart in areas with high tortoise density. A camera trap was set facing the rails (one camera/trench) in May–August 2016 (184 trap days, 92/camera). Individual tortoises were identified using a combination of size, shell patterns, shell shape, and forelimb scale patterns.

    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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