There is a series of jokes about how the chicken crossed the road. The simple answer: with difficulty. That seems also to apply to bats.
In the UK , a series of papers by Altringham et al showed that bats don't readily adapt to culverts put in for them: most carried on crossing the line of the now missing hedges that were there before the new roads were built. The result? Mortalities.
In a study in S Australia, Bhardwaja et al have looked at bats crossing roads via underpasses and open span bridges. The idea was that bats would use these, rather than launch into the traffic. Did they? They looked at the use of open-span bridges and box-culverts. Bats that normally fly close to vegetation crossed the highway more under bridges than through culverts or by going over the road. Open area bats largely ignored structures, crossing the highway over the road.
Bhardwaja et al looked at bat activity under and above 6 open-span bridges, 6 box culverts and 6 unmitigated sites along a major highway in Australia. After a bit of mathematical chicanery (Poisson regression models within a Bayesian framework) they compared the activity of 12 bat species (categorised as clutter-adapted, edge-adapted, and open-adapted species) under the structures, over the road above the structures, above unmitigated segments of the highway, and in the vegetation next to the roads.
The results: bats were less active above the road than they were in the surrounding vegetation or under bridges. Seven species crossed the highway more under bridges than they did through culverts or by going over the road. This backs up Altringham et al , who suggested that culverts were of limited value. So, bats will cross, but not go where you tell them. Clearly, spending lots of money on tunnels may be a foolish option without a little bit of ecological assessment and field work.
M. Bhardwaja, , , K. Soanesa, b, , T.M. Strakaa, , J.J. Lahoz-Monforta, , L.F. Lumsdenc, , R. van der Reea, d, Show more https://doi.org/10.1016/j.biocon.2017.05.022