American movie… early 1970’s

Last scene: Two or three people in a van or 4wd on the highway. They’ve committed some crime and think they’ve gotten away with it. They get stopped by a highway patrolman on a motorbike. He just checks the driver’s licence then lets them go on their way.

They start driving away but the patrolman starts following them on the motorbike. He is waving to them. It turns out he accidentally forgot to give the driver back his licence and just wants to return it.

The crims think he’s onto them. One of them goes into the back of the van, rolls down the window at the back, then blasts the patrolman with a shotgun, Patrolman falls off the motorbike in slow motion. then the credits roll.