M4 crash: Unravelling the moment car drove wrong way
A wave of desperate 999 calls reported a car travelling the wrong way along the M4 motorway one February night.
Help was not able to arrive in time to stop it ending with one driver dead and another seriously injured.
The BBC Crash Detectives series explores how police unravelled what had happened.
Forensic collision investigator Dean Burnett said: "If you see the cars swerving out of the way... you can only imagine what those drivers are going through."
The story of how police pieced together what happened was shown in The Crash Detectives on BBC One Wales and is available on iPlayer.