Two men posing as police officers pulled over a driver and attempted to fine them for "non-essential travel".
The pair, who wore ear pieces and carried walkie-talkies, stopped a woman on the Leven bypass in Beverley, East Yorkshire, said police.
When she refused to pay an "on-the-spot cash fine" of £60, the offenders drove off.
Humberside Police said it was not stopping vehicles in response to new coronavirus legislation.
Ch Supt Darren Downs said fines were a "last resort", and would never be issued on-the-spot.
He added: "There are no check points or stop areas in place, and drivers will not be pulled over by officers to just ask where they are going, unless it is in response to a specific event."
The force urged motorists to ask for identification if stopped by someone they suspect of pretending to be an officer.