High Wires, Low Evidence: The Unsettled Case for Bat Crossings on South West Roads

At dusk, the A30 and A38 can seem to offer two different landscapes at once. To drivers, they are corridors of speed and light. To bats, they are interruptions: wide bands of warm, noisy, illuminated ground that some species will not readily cross. In recent years, road schemes in Devon and Cornwall have responded with structures intended to reconnect those severed flight paths, from wire gantries strung above carriageways to culverts and underpasses beneath them. The intention is sound. The difficulty lies in what happens after the ribbon is cut and the mitigation is signed off.

A structure that looks like a solution

Bat bridges, as they are commonly called, are not bridges in the conventional sense. Many are gantries of wire or rope suspended across a road, sometimes with planting or latticework intended to mimic the edge of a hedgerow or tree line. The theory is that certain bat species will follow linear features and use the structure as a safer route above traffic. Culverts and underpasses work on a different principle, offering a ground-level route beneath the road for species that will use them.

On paper, these interventions fit neatly into the mitigation hierarchy that governs English road planning: avoid, mitigate, compensate. Where a new dual carriageway cannot avoid a wooded valley or a known commuting route, a crossing structure becomes part of the scheme’s ecological package. It is tangible, photographable and easy to include in a planning condition. That is precisely why it deserves closer scrutiny. A structure that looks like a solution can too easily be accepted as one.

What the counts can and cannot show

The evidence base for bat crossings is more equivocal than the confidence of scheme documents sometimes suggests. Post-installation monitoring is not uniform across schemes. Some studies use acoustic detectors to compare bat activity near the structure with activity elsewhere; others rely on carcass surveys, which are blunt instruments for animals as small and cryptic as bats. Detector placement, weather, season and species mix all affect what is recorded. A burst of calls near a gantry does not prove that bats crossed above the traffic, still less that the structure reduced mortality.

There is also a timing problem. Ecological monitoring is often commissioned for a short period after construction, when the surrounding landscape is still settling and before longer-term patterns can emerge. A two- or three-year window may capture use of a culvert by a determined species, but it is a weak basis for judging whether a gantry has restored functional connectivity across a road corridor. In the South West, where road schemes often cut through mosaics of pasture, woodland and hedgerow, the relevant question is not simply whether bats are present. It is whether they can move between roosts and foraging grounds in a way that sustains the local population.

The risk in the road scheme ledger

Road schemes assess ecological risk through environmental impact assessment, protected species licensing and, increasingly, biodiversity net gain. Each has its place. But bats expose a gap between the arithmetic of habitat compensation and the ecology of movement. A scheme can deliver a measurable gain in habitat units while still leaving a commuting route severed. A gantry can be listed as mitigation in a condition and then treated as completed once the wires are tensioned.

This matters because road investment is long-lived. A dual carriageway built in the 2020s will shape the landscape for decades, and the associated mitigation will either help or hinder wildlife for just as long. In 2026, with pressure on public spending and a growing emphasis on evidence-led environmental policy, the case for rigorous post-construction monitoring is not a luxury. It is the difference between adaptive management and an accounting exercise.

Toward a more honest standard

A more credible standard for bat crossings would begin before construction. Success criteria should be set in advance and tied to species and landscape function, not simply to the presence of a structure. Monitoring should run for longer than the usual short-term window, be independent of the scheme promoter, and be published in a form that local records centres and future scheme designers can use. Where a structure fails to meet its criteria, there should be a clear route to remedial action rather than a quiet acceptance that bats have not taken to it.

Design also needs to be more honest about uncertainty. Not every road crossing can be made permeable for every species. Retaining hedgerows and dark corridors, avoiding unnecessary lighting, and designing underpasses at appropriate scale may sometimes achieve more than a conspicuous gantry. The best mitigation is often the least photogenic: a route that keeps bats away from the road in the first place.

The South West’s road network will continue to be renewed and, in places, expanded. Each scheme brings a familiar promise that ecological harm will be mitigated. Bat crossings are part of that promise. They should be judged by whether they work, not by whether they make a difficult decision look complete.