Bat bridges are purpose built structures, typically wire gantries, built over roadways to assist bats in navigating safely. Since bats rely on echolocation and follow linear features such as hedgerows, these bridges encourage them to fly at greater heights, reducing the risk of collisions with vehicles following habitat destruction. The theory is elegant. Bats navigate using sonar. They follow hedgerows. Motorways destroy hedgerows. Install wire structures that look like hedgerows to bat echolocation. Bats fly over traffic instead of through it. Problem solved.
Except the bats never got the memo.
The £2 Million Failure
The 15 bat bridges across the UK from Cumbria to Cornwall cost £2 million to the taxpayer, at more than £140,000 a time. Wire bridges built to guide bats safely across busy roads simply do not work, University of Leeds researchers confirmed in a 2012 study. A replicated study at four bat gantries on roads in northern England found fewer bats using gantries to safely cross roads than crossing below them at traffic height, with only 1 to 11 percent of crossing bats using the structures while 17 to 84 percent crossed at traffic height below the gantries.
The Leeds team monitored four wire bridges spanning major roads in the north of England. All had been built over the last nine years to replace hedgerows when these routes were severed by new roads. Almost all the bats in the study favoured their former commuting routes rather than the wire bridges, crossing the road at a low height. There was no evidence to suggest that the bats changed their behaviour in response to the bridges, even over time.
A well established wire bridge built nine years ago and only 15 metres from the severed commuting route it replaced was still spurned by the bats. Nine years. Fifteen metres from where the bats wanted to go. They ignored it completely and kept flying through traffic.
Why They Don't Work
The idea is that they encourage bats to fly higher, out of the way of passing vehicles. When the ground is lowered for motorways, these things are built above. Bats detect it with their sonar as if it were a hedgerow. That's what's supposed to happen. What actually happens is bats continue using their original routes regardless of whether those routes now cross motorways at bumper height.
Unfortunately, poor design or poor connectivity to the bats' own flyways along streams, hedges and woodlands has meant that few attempts have been successful. Poor monitoring, or a complete absence of monitoring, has meant that this failure has gone undetected, and we have continued to build structures that don't work.
Dr Anna Berthinussen, a bat ecologist, said the evidence suggests that actually no, these structures are not effective, they're not meeting their purpose. The government commissioned study that reached this conclusion focused on seven bridges built across the Norwich Northern Distributor Road. Government and council chiefs nevertheless continued building more bridges after the research proved they didn't work.
The One Success Story
At one underpass 96 percent of bats flew through it in preference to crossing the road. Finally, success. Except this underpass was located on an original bat commuting route. The bats used it because it was built exactly where they already flew. At two other underpasses, attempts to divert bats from their original commuting routes were unsuccessful and bats crossed the road at the height of passing vehicles.
The lesson is clear. Build on existing routes and bats will use the structure. Try to convince bats to change their behaviour and they'll ignore you completely while flying into traffic.
What Should Have Been Done Instead
Wire bat gantries, of the type studied, should not be used, and attempts to divert original commuting routes should, if possible, be avoided. Underpasses built on existing commuting routes can be effective crossing structures, if commuting bats can maintain their original course and flight height. Further investigation into more substantial, natural crossing structures over roads, such as green bridges, and simpler options such as tree hop overs, is needed.
Professor John Altringham, who led the University of Leeds research, said we should certainly stop building wire gantries. That was in 2012. The government kept building them anyway.
The Political Backlash
The overall cost of bat bridges was criticised by Lord Marlesford in the House of Lords in 2011, for being funded at a time when we're having to cut a lot of public spending. Spending £140,000 on structures that provably don't work while cutting public services didn't sit well politically. Spending £2 million total on them sat even worse.
A Reddit user explained: We keep destroying hedgerows and the bat bridges were an attempt to mitigate for habitat loss. I don't know how effective they are, but they weren't put up there for fun. The intentions were good. The execution was terrible. And the monitoring was insufficient to catch the failure before millions were spent.
What Actually Works
Crossing height was strongly correlated with verge height, suggesting that elevated verges may have some value in mitigation, but increased flight height may be at the cost of reduced permeability. Simply raising the road verges might help more than elaborate wire structures. Or just building underpasses where bats already fly instead of trying to convince them to use new routes.
The uncomfortable truth is that bats evolved for millions of years following linear features at low height. They're not adapting their behaviour to accommodate motorway construction on human timescales. Roads act as barriers to bats, cutting colonies off from established feeding sites and reducing their ability to feed themselves and their young. Wire bridges don't solve that problem. They just create the illusion of solving it while bats keep dying.
So next time you see those weird wires over a UK motorway, remember you're looking at £140,000 worth of good intentions and ecological misunderstanding. The bats are still crossing below them, at bumper height, wondering why humans built elaborate structures fifteen metres away from where they actually fly.