You brake, wondering what on earth that was, but in the dim glow of the rear lights, it’s impossible to see.

That’s often all motorists know about it when they hit an otter. This most beloved of Scotland’s mammals has increased in numbers in the past 20 years, with Scottish Natural Heritage estimating that there are now 8000 Scotland-wide – a healthy population, though not yet at capacity.

But in spite of the installation of otter underpasses on some roads, a worrying number are still being killed under the wheels of cars and lorries, according to Scottish Natural Heritage. The three roads with the most recorded deaths are the A75 from Gretna to Stranraer, the A9 from Polmont to Scrabster and the A87 from Invergarry to Skye.

Rosemary Green, formerly of the Vincent Wildlife Trust, is Scotland’s foremost authority on otter road deaths and has been collecting data on the subject for many years.

In one study she carried out on the A75, she found 50 dead otters on the road in a year. Since then, some improvements have been made, but she says the road is still a problem. On one recent drive along it, she saw a dead otter, a dead badger and a dead fox.

“It’s long, it crosses a lot of river systems and drivers go fast along it,” says Ms Green, by way of explanation. “Otters don’t have much road sense. I’ve several times stopped and chased them off the middle of roads, including, once, a courting couple.

“But the real problem is the speed that drivers go at.”

There is a healthy popu­lation of otters in the north and south-west of the country and they are also returning to much of central and south-east Scotland including the Clyde in Glasgow and the Water of Leith in Edinburgh, mainly because of improvements in water quality.

However, conservationists fear this recovery could be impacted by road deaths.

Otter underpasses beneath roads are an established way of trying to reduce roadkill – and when introduced properly can almost eliminate deaths in blackspots, according to SNH’s officer for East Dumfriesshire, Jonathan Warren.

The A7 south of Langholm passes through a delightful landscape of rolling hills and wooded dells.

Recently, SNH advised contractors on the building of otter culverts beneath the Auchenrivock bypass.

The brand new road, which opened in June, runs along a hillside and crosses a number of burns that are otter highways.

The contractors have installed designer culverts to give the local otters safe passage beneath the road.

One such culvert is located next to a local access road. It is a concrete tunnel, a metre or more wide, providing a way through for the burn – and along its length is an otter pavement.

Warren hunkers down next to it. “This culvert is to carry the peak storm flow. If it’s low, they can walk through; if it’s half full, they can swim through it,” he says. If it’s much higher, they won’t be able to get through.

He points behind him. “That’s why we have this other pipe higher up.

“To the left is a 60cm diameter pipe, big enough for badgers and otters, which remains dry at all times.

Installing such pipes is not cheap, but is a lot cheaper at the design stage than after the road is complete.

This is only half of the job, though. There is also a painstakingly otter-tight system of fencing keeping the animals away from the road. The tight-mesh barrier begins about 20m away from the culvert, runs above it and joins up with a dry stone wall on the other side. It is dug in 30cm below ground so nothing can tunnel underneath. Although the otters could just walk to where the fencing begins and on to the road, the location of the barrier is based on assessments of the animals’ predicted route through the landscape – alongside water. Before the road was improved, there were reports of otter and badger deaths; in the past three months, there have been none. “We hope no otters or badgers will appear as casualties on this road; that’s our aim,” says Warren.

Such mitigation measures have to be carefully planned, though, or they can actually make the problem worse. Ms Green cites an instance of otter-proof fencing being put on one side of a road but not the other, so otters became trapped on the road.

Warren also recalls one scheme where otter culverts and fencing were put in, and a dead otter was found just two weeks after the road opened. He then examined the fencing and found a gap between two sections; it was swiftly closed.

“It does work, otherwise we wouldn’t be doing it,” he says.

However, it is perhaps drivers who must now do their bit to prevent otter road deaths. Ms Green’s message is simple: “Don’t drive so fast, especially on cold, wet, windy nights, for your own safety as well as that of wildlife.”