Drivers were urged to get a grip during spot checks by police at a car park in Brighton.
And to try the 20p test to make sure that their tyres were safe.
Sussex Police said: “Roads policing officers have warned drivers to make sure their tyres are fit for use after carrying out checks.
“Officers visited Churchill Square shopping centre car park and over four hours examined the tyres of more than 400 vehicles that had been left there by motorists on Sunday 21 December.
“A total of 13 vehicles had defective tyres, most of which had tread of less than 1.6mm. Of the 13 vehicles, four had two defective tyres.
“The vehicles’ details were recorded and Tyresafe leaflets were left for the owners as well as Sussex Safer Roads Partnership forms advising them that their tyres needed to be changed immediately.”
PC Phil Barrow said: “We chose Churchill Square because it allowed us to check far more vehicles than if we had been stopping vehicles by the roadside.
“It also meant that we did not have to unnecessarily delay any motorists whose vehicles were in a good condition.
“Having good tread on your tyres can be the difference between your vehicle gripping the road and it not, when you drive on to a slippier part of the surface.
“It could be what prevents you from skidding off the road or into another motorist and causing a serious crash.”
Sussex Police said that tyres should ideally be checked at least once a month and before any long journeys, adding: “This should include the air pressure, overall condition and that the tread depth is at least 1.6mm across the central three quarters of the tyre and around the whole circumference for cars.
“A quick way to check the tread is to use a 20p piece.
“This is done by simply inserting a 20p coin into a tyre’s main tread grooves to see if the outer rim of the coin is covered by the tread.
“Repeat this in all the main grooves and at several points around its circumference.
“If you can see the rim of the coin when it is inserted at any point, the tyre may be illegal and should be checked by a tyre specialist.”
Further unannounced tyre checks are planned for January and throughout the year.
The force said that people in Sussex could text officers on 65999 with details of vehicles that were being driven while in a dangerous condition or report them at www.operationcrackdown.co.uk.