Action Synopsis: Bat Conservation About Actions

Install overpasses as road/railway crossing structures for bats

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • Four studies evaluated the effects of installing overpasses as road crossing structures for bats. Three studies were in Europe and one in Australia.


  • Richness/diversity (1 study): One site comparison study in Australia found that the same number of bat species were recorded at an overpass and in nearby forest and bushland.



  • Use (3 studies): Two replicated studies (including one site comparison) in Ireland and France found that two or three bat species/species groups used overpasses but up to three-quarters of bats crossed the road below at traffic height or crossed at other nearby locations. One study in the UK found that an overpass with planters was used by two-thirds of crossing bats, and an unvegetated overpass with a paved road over it was not used by crossing bats.

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, site comparison study in 2008 at six overpasses over a road in agricultural and woodland habitat in southern Ireland (Abbott et al 2012) found that three bat species or species groups flew over overpasses but 39–75% of activity was recorded over the road below them, and lower activity was recorded on overpasses than in adjacent habitats. Overpasses were used by three bat species or species groups (common pipistrelle Pipistrellus pipistrellus, soprano pipistrelle Pipistrellus pygmaeus, Myotis spp.), but 39% of common pipistrelle passes, 49% of soprano pipistrelle passes, and 75% of Myotis spp. passes were recorded over the road below overpasses. Bat activity was lower (by >10%) on overpasses than in adjacent habitats (data reported as statistical measures). The overpasses (8–11 m wide x 6–11 m high x 58–76 m long) had minor roads over them. The motorway (65–70 m wide) had four lanes carrying an average of 20,000 vehicles/day. Bat detectors recorded bat activity above and below each of the six overpasses and simultaneously at two adjacent linear features on two nights in May–September 2008.

    Study and other actions tested
  2. A study in 2013 at two overpasses over two roads in the UK (Berthinussen & Altringham 2015) found that an ‘environmental’ bridge was used by almost two-thirds of crossing bats, but an unvegetated overpass carrying a paved road was not used by crossing bats and only three bats crossed the road nearby. A greater number of bats used an ‘environmental’ bridge (62%, 118 of 192 bats) than crossed the road below at traffic height (19%, 36 of 192 bats). Six bat species or species groups were recorded in total (see original report for data for individual species). An unvegetated overpass was not used by crossing bats and only three bats were observed crossing the road nearby (12–20 m away) at heights of 2–20 m. Both overpasses were designed as crossing structures for bats, alongside other purposes. The ‘environmental’ bridge (30 m long x 5 m wide x 6 m high) had solid vertical sides (2 m high) and was covered with deadwood and planters of hawthorn Crataegus monogyna. The overpass (40 m long x 15 m wide x 8 m high) had a paved road over it and no vegetation. It was designed to carry traffic and provide a crossing structure for bats. Observations of crossing bats and recordings of bat calls were made during 6 x 60-minute surveys at dusk or dawn at each overpass and the road below in June–August 2013.

    Study and other actions tested
  3. A site comparison study in 2014–2015 of one vegetated overpass over a road within a forest reserve in Brisbane, Australia (McGregor et al 2017) found that the overpass had higher bat activity but the same number of bat species as adjoining forest and bushland. More bat passes were recorded on the overpass (average 52 bat passes) than in the adjoining forest (27 bat passes) or bushland (29 bat passes), although no statistical tests were carried out. Nine bat species were recorded on the overpass and in the adjoining forest and bushland (see original paper for data for individual species). The overpass was hourglass shaped (70 m long x 15 m wide at the midpoint and 20 m wide at the ends) and was planted with natural vegetation and mature saplings (70 shrubs and six trees/100 m2). It was built over two dual lanes of a major urban road bisecting forest and bushland. Bat activity was recorded using bat detectors over two consecutive nights/month between December 2014 and July 2015 at two stationary points on the overpass and along eight 75 m transects perpendicular to the road.

    Study and other actions tested
  4. A replicated study in 2016 of two overpasses over a road near Lyon, France (Claireau et al 2019) found that both ‘U-shaped’ metal overpasses were used by two bat species or species groups, although 51–65% of bats crossed the road at other locations. One overpass, installed within a bat commuting corridor, was used by 39% (26 of 67) of crossing bats, whereas 51% (34 of 67) crossed the road at other locations within the corridor. The other overpass, installed 325 m from a bat commuting corridor, was used by 19% (7 of 37) of crossing bats, whereas 65% (24 of 37) crossed the road within the original commuting corridor. Crossing heights were not reported. Two bat species/species groups (all Pipistrellus spp.) used the overpasses and 4–6 bat species/species groups crossed at other locations (see original paper for details). Both overpasses (40 m long x 4.8 m wide, <10 m high) were installed (2.5 km apart) over a new four-lane highway in November 2012. At each site, six pairs of bat detectors on opposite sides of the road (one pair at the overpass, five pairs within the commuting corridor or adjacent habitats) simultaneously recorded crossing bats over five consecutive nights in July or September 2016.

    Study and other actions tested
Please cite as:

Berthinussen, A., Richardson O.C. and Altringham J.D. (2021) Bat Conservation: Global Evidence for the Effects of Interventions. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.


Where has this evidence come from?

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Bat Conservation

This Action forms part of the Action Synopsis:

Bat Conservation
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