Killer in the Backseat

Killer in the Backseat (also known as High Beams) is a common, car-crime urban legend well known mostly in the United States and United Kingdom.

The legend involves a woman who is driving and being followed by a strange car or truck. The mysterious pursuer flashes his high beams, tailgates her, and sometimes even rams her vehicle. When she finally makes it home, she realizes that the driver was trying to warn her that there was a man (a murderer, rapist, or escaped mental patient) hiding in her back seat. Each time the man sat up to attack her, the driver behind had used his high beams to scare the killer.
In some versions the woman stops for gas, and the attendant asks her to come inside to sort out a problem with her credit card. Inside the station, he asks if she knows there's a man in her back seat. In another she sees a doll on the road in the moors, stops, and then the man gets in the back.

The story is often told with a moral. The attendant is often a lumberjack, a trucker, or a scary-looking man; someone the driver mistrusts without reason. She assumes it is the attendant who wants to do her harm, when in reality it is he who saves her life.

Looks like we've got a good Samaritan