Traffic police in Lancashire have said an increase in car passengers putting their feet up is risking lives.
Officers said they have noticed more and more front-seat passengers resting their feet on the dashboard.
Front air bags in cars make the practice even more dangerous if the vehicle was involved in a crash.
Pc Ian Brayshaw, of the motorway unit, said that air bags deploy at 200mph, can cause serious injury if the passengers' legs are raised up.
"The majority of vehicles are now fitted with front air bags and most seat belts have a pre-tensioning system to hold people in place in the event of an impact," he said.
"If someone were sitting in the front seat with their feet on the dash and the car hit something, both these safety features would deploy.
"An air bag deploys at more than 200mph, the force of 6gs. That's three times as much as a jet pilot is subjected to.
"The injuries to that person's legs could therefore be catastrophic - at least broken ankles, possibly severed legs, maybe even fatal injuries."