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Adder underpasses: Wildlife trust pioneers UK’s first use on roads
Rod Sweet Global Construction Review
The specially-designed snake tunnels link two isolated populations at Berkshire’s biggest open heath.
Adders are the UK’s only native venomous snake, but they’re extremely shy and adder-bite deaths are rare. Image: Charles J. Sharp/CC BY-SA 4.0
Adders are one of only three species of snake native to the UK, and surveys show that 90% of monitored populations are shrinking. Many groups have fewer than 10 individuals. Habitat loss is thought to be the cause.
They are the UK’s only venomous snake, which, for some, may engender ambivalence about their survival.
But they’re extremely shy and, in the last 150 years, there have been only 14 recorded deaths from adder bites, the last in 1975, according to the Amphibian and Reptile Conservation Trust.
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In the UK, adders are protected by law. Preying on small mammals, they help keep ecosystems such as heathland in balance.
At Greenham and Crookham Commons, the biggest continuous tract of open heath in Berkshire, radio tracking between 2015 and 2022 showed that there were two distinct adder populations, kept apart by the Old Thornford Road.
Now, though, thanks to a project organised by the Berkshire, Buckinghamshire and Oxfordshire Wildlife Trust (BBOWT), two 10m-long snake tunnels – adder underpasses, let’s call them – connect the two populations safely so they can visit each other.
One of two 10m-long snake tunnels that safely connect the two isolated adder populations so they can visit each other. Image: ACO Water Management
This matters because genetic isolation makes species more susceptible to mutations and diseases that can lead to extinction.
Snakes and roads don’t mix
Roads mean death for adders, either from being squashed by cars or snatched for mauling by crows, buzzards and even pheasants.
In 2023, BBOWT received £113,000 from Natural England’s Species Recovery Programme to build the underpasses. It’s the first time such an approach has been tried in the UK.
ACO Water Management supplied two specially-designed tunnels with wing walls to direct the adders toward the entrances.
The tunnels let sunlight in, important for cold-blooded reptiles, as well as fresh air and moisture. Removable grates make inspecting and cleaning easier.
They were installed in 2024, lined with a gravel substrate and safe points where adders can hide from predators.
The areas on either end were cleared of shrubs and bushes to create basking areas.
Roads mean death for adders, either from being squashed by cars or snatched for mauling by crows, buzzards and even pheasants. Image: ACO Water Management
Can’t be bothered in winter, but …
In autumn 2024, the trust tagged five adders with small radio transmitters to track their movements.
With the winter hibernation approaching, the trust didn’t expect much movement, although some were found close to the tunnel entrances.
Cameras showed the tunnels to be popular with rodents, small birds and grass snakes but not adders, yet.
The Adder underpasses let sunlight in, as well as fresh air and moisture. Removable grates make inspecting and cleaning easier. Image: ACO Water Management
The breakthrough came with the sun’s return the following spring, in April 2025, when a camera filmed an adder using the tunnel to get from Crookham Common to Greenham Common.
We can’t know how that visit went off camera, but it showed that the two isolated groups could now interact, a potential step toward resilience in the local population.
The trust will continue monitoring the populations to see how it all goes.
“It’s been fantastic to be able to contribute to a project that has such a clear and beneficial impact on the local biodiversity,” said Terry Wilkinson, specialist design and application engineer at ACO.
“We expect the tunnels we’ve supplied to last for many years and hope to see the adder population return to healthier numbers now that they can move more freely, without having to cross a dangerous road.”
Debbie Lewis, the trust’s head of ecology, said: “The ACO tunnels were perfect for the job, being robust and easy to install.”
