Car dealerships across Essex are being warned against adding tinted film to windscreens and windows after a recent spate of prosecutions.
Essex Police say the tints laid by some retailers take light-entry levels below legal limits of 70-75% and increase the risk of road collisions.
Drivers could face prosecution and fines for dangerous driving if caught with illegally-tinted vehicles.
A police spokesman said anything that restricts vision must be dangerous.
The law says the front windscreen has to allow through 75% of available light and the two front side windows must allow through 70% of light.
Road policing Insp Nick Burston is hoping both retailers and drivers heed the warning rather than risk prosecution.
"There are a number of issues here and a lot of decent people may not realise that they are breaking the law.
"We're not picking on motorists. We just want to make sure drivers are aware of this issue so that they don't find themselves in unnecessary bother. It's not fair to motorists if they don't know the rules," he added.
Drivers could face a fine of £60 as well as three points on their licence or prosecution for dangerous driving.