https://www.bbc.co.uk/iplayer/episode/b ... -episode-2
A motorcyclist is found dead on the central reservation of the M4. Piecing together his final moments proves to be a painstaking challenge.
David Evans had been working the Christmas Day shift monitoring CCTV at a power station on the south Wales coast.
In a gesture of seasonal goodwill, a colleague had told him to leave early and get home to his family; he'd take over from here.
As Mr Evans, 49, set off on his Honda CB 500 motorcycle for the hour-long ride home in the wind and rain, his wife and two young daughters were putting the finishing touches to Christmas dinner, awaiting the familiar rumble of his bike pulling up outside.
The call came through to the police control centre 40-or-so minutes later at about 18:00 from a member of the public.
"Just before junction 24 eastbound on the M4," he said. "There's a motorbike right against the central reservation… It's actually on the floor. But there's no sign of the driver."
Bob Witherall, a forensic collision investigator, was with his family finishing his turkey dinner at home when he heard the report go out over his police radio.
When he arrived at the scene the motorway was shut, a stream of backed up headlamps stretched further into the distance by the second, frustrated motorists sighed and drummed their fingers on steering wheels. They had places to be - the long-awaited sanctuary of home, family and friends to visit, Christmas dinners to be eaten.
He was told the motorcyclist had been located.