The Highways Agency tracked tagged dormice
Dormice in Devon and Cornwall are crossing some of the region's busiest roads - to make their home in the central reservation.
Conservationists say the tiny nocturnal animals are happily living and breeding on verges in the middle of roads such as the A30 and A38.
To make them more comfortable, nest boxes are being placed at some sites.
Experts spotted the animals after 50 were electronically tagged for a study into their movements.
The creatures have been found on dual-carriageway reservations at several sites, including Bodmin and Okehampton, the Highways Agency study found.
The agency has been monitoring the protected species to see if the A30 and A38 were affecting population numbers.
Dormice normally inhabit woodland with scrub and hedgerows.
They are good climbers, and often spend most of their time in the tree canopy, preferring to travel from tree to tree without having to cross the ground, the Devon Wildlife Trust said.
Alison Sixsmith, from the agency, said: "We weren't aware that they would be able to cross such a stretch of road to get to a central reservation.
"The fact that they have is fantastic because it means the road isn't the imposing barrier we thought it was."
'Unlikely to travel'
She said that it was not certain how many of the nocturnal animals originally tried to make crossings or how many might have been killed by traffic.
But she added that, once established, the animals were unlikely to move and face the dangers of the road again.
She said: "Once they're there, they are unlikely to travel.
"They get to a habitat where they have everything they need, where they have suitable food and stay there and breed."
Ms Sixsmith said measures were being taken to ensure that dormice stayed in their current nests.
She said: "Where we know we have that we have creatures such as dormice, which are protected by European law, we make sure that we keep an eye out on them."
Four sites are being monitored across Devon and Cornwall, and so far the agency has provided about 250 nest boxes, it said.