Dig trenches around culverts under roads/railways
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
Background information and definitions
Collisions with vehicles can be a large cause of mortality for mammal species (e.g. Forman & Alexander 1998). Underpasses installed beneath roads or drainage culverts may be made accessible to mammals with the intention of increasing connectivity of habitats and reducing the animal-vehicle collision risk associated with crossing the road. A range of means may be employed to help funnel animals towards such crossing points. These are usually fences or similar barrier features to prevent animal crossings. However, trenches may be dug at some sites with the intention of inhibiting crossings, especially of small mammals.
See also: Transportation and Service Corridors: Install barrier fencing along roads.
Forman R.T.T & Alexander L.E. (1998) Roads and their major ecological effects. Annual Review of Ecology and Systematics, 29, 207–231.
Supporting evidence from individual studies
A replicated, randomized, controlled, before-and-after study in 2015 along a road through dry savanna in Limpopo, South Africa (Collinson et al. 2017) found that digging trenches alongside culverts did not reduce the number of mammals killed on roads. Results were not tested for statistical significance. One mammal (a South African pouched mouse Saccostomus campestris) was detected as a roadkill near culverts after trenches were dug and one (a red veld rat Aethomys chrysophilus) was found before they were dug. Over the same period, near culverts where no trenches were dug, two multimammate rats Mastomys sp. were detected as roadkills after trenches were dug at treatment sites and one was found before trenches were dug. The study was conducted in January–February 2015 along 400-m-long road sections with 2-m-wide culverts. In three sections, a 30-cm-deep trench, 2 m from the road verge, was dug for 200 m on either side of the culvert. Three road sections had no trench. Roadkills were counted at all sites over 20 days before the trench was dug and 20 days afterwards, by an observer in a car moving at 40–50 km/h.Study and other actions tested