A city sanitation truck caused a three-car crash Saturday night on the Belt Parkway in Queens — where commercial vehicles are not permitted — when it smashed into the Merrick Boulevard overpass.

The compactor on the back of the truck came loose and crushed two cars behind it, injuring four people, one critically, according to police and radio transmissions from the scene.

The injured were taken to Jamaica Hospital where all were expected to survive, police said.

Calls to the Sanitation Department were not returned Saturday night.

Additional reporting by Danielle Furfaro