A HIT police TV documentary series has revealed how a poor road surface on a notorious A40 slip road was to blame for a Swedish duo crashing their motorbike into the central reservation barrier, leaving them with serious injuries.

BBC One’s The Crash Detectives reveals that the BMW R 1250 GS Adventure bike sent out an automated SOS call when it careered out of control at the A449 junction near Raglan in May 2022, enabling 999 responders to rush to the scene.

Swedish duo Johan Takats and Helena Engstrom were too badly injured to respond to a call from BMW's emergency centre after the bike detected the impact, but the latter were able to pinpoint the machine's location through GPS and alert Gwent Police.

"Due to the intervention of the technology on the motorcycle, detecting the collision, and making that call, it's probably got help to them more quickly than if they were having to describe their location," says forensic collision investigator PC Richie Wyatt.

And examining the scene, he reveals: “Because of the curvature of this road, I’ve been here several times for collisions of different severities.”

The duo were taking a road trip with two other riders, and had travelled through Germany to Amsterdam, across to Newcastle, down to Aberystwyth via Hadrian's Wall, and were on the way to Stonehenge and then France.

IT worker Mr Takats, 47, told BBC Wales News: "It is nice to know that in an accident the bike can send information like location, speed and direction of travel to the call centre, though I never thought I would be needing it."

In the episode, PC Wyatt reconstructs the crashed bike's movement and is able to rule out excessive speed.

But his investigations reveal that the road surface on the inside lane on the tight left-hand bend was potholed and gravel debris had spread onto the outside lane.

"Probably the one vehicle you don't want to be on, to travel through that debris and that is a motorcycle," he says.

"To have that amount of potholing and grit on the road surface at that time and for that to be a sole causation factor of the collision is relatively rare."

Mr Takats can't remember much about the accident, but admitted: "Our helmets saved us, both of them were really damaged.”

Pillion passenger Ms Engstrom told BBC Wales News: "I remember being in the air, going over the barrier and landing on the road on the other side.

"I was thinking I needed to roll back against the barrier so I wouldn't get hit by oncoming cars."

Both of them, who live near Malmo, needed surgery and spent two weeks at the University Hospital of Wales in Cardiff before more treatment in Sweden, and spent a year recovering.

Mr Takats said: "We are very grateful for all the help we received... We would like to say a big thank you to all of them."

And they are back riding, having toured to Croatia this summer.

Catch The Crash Detectives (series 5): Fair to all Parties on BBC iPlayer.