In the Netherlands, a steady increase in the density of roads, among other factors, contributed to a crash in badger Meles meles numbers in the 1970s. As a consequence, badger underpasses were included in the construction of new roads. A review was undertaken to identify the best badger underpass designs based on 30 years of Dutch experience.
Over a period of 30 years, numerous surveys were undertaken to assess the efficacy of different badger underpass designs; more than 200 underpasses were constructed. Experiments included trials using fencing (of various length and design) aimed at preventing access to roads whilst leading animals to the underpass entrances; the efficacy of unfenced ‘control’ passages for comparison were included.
Passage design: Badgers preferred simple tubes (diameter of 30-40 cm) with a slightly rough inner surface. Concrete pipes were effective and highly durable; (Armco) steel provided an alternative where ground subsidence was expected. This design was successfully used by badgers to pass beneath roads, even if over 100 m in length. A cornice fitted to the entrance helps prevent water from entering if the passage lies below ground level.
Fence design: Underpass effectiveness was highly dependent upon the quality and the length of fencing either side of the underpass entrance. Experience indicated galvanised, spot-welded wire mesh netting (mesh size 2.5 x 5 cm) to be a good fencing material. Fencing around 1 m high (posts positioned on the road-facing side) proved effective in preventing badgers from climbing over; digging the fence 20 cm deep into the ground with a 30 cm wide, horizontal strip of mesh attached to the bottom prevented them from burrowing underneath.
The longer the fence the more effective the underpass, with a notable decrease in effectiveness for fences less than 250 m long. Integrating one-way flaps allowed any animals on the road-side of the fence to pass through.
Defects in fences (e.g. holes allowing animals through) reduced the number of underpasses used by nearly 50%; regular fence inspections are required with defects promptly repaired; fences need replacing every 15–20 years. Surveys further revealed that 35-40% of the underpasses were not used by badgers at all. As well as ensuring fence maintenance, passages appeared more effective when positioned near existing badger crossing points.
Badger mortality on roads: Despite the construction of more than 200 underpasses, the numbers of badgers killed rose annually by more than 12% during 1990-95 (from 216 in 1990 to 273 in 1995). However, both the density of traffic and the number of badgers increased, resulting in a higher probability of accidents.
Use by other mammals: Red fox Vulpes vulpes, stone marten Martes foina, polecat Mustela putorius, weasel Mustela nivalis, American mink Mustela vison, hedgehog Erinaceus europaeus, rabbit Oryctolagus cuniculus, cat Felis catus, and a number of small rodents, also used the underpasses.
Note: If using or referring to this published study, please quote the original paper. This German language paper, translated and summarised for Conservation Evidence, has an English and French abstract, and is available at: http://www.springerlink.com/content/y26vt424w4475126/fulltext.pdf