Police in Greater Manchester are taking action against more than 300 drivers a month for having tinted windows on their cars.
Tinted windows at the front are illegal
Over-tinted windows are illegal and seriously restrict drivers' views of the road, the force says.
Since October 2006, GMP has issued 1,928 fixed penalty notices and 440 prohibitions on offending drivers.
Officers use special equipment to measure the amount of light passing through the windows.
Manufacturers already tint glass to the maximum amount allowed, so any tint added automatically makes the windows illegal.
Pc Dave Barrow, of GMP's traffic network section, said: "Tinted windows are illegal for a reason - they are unsafe.
"I would urge anyone considering them to have a serious re-think, because we will penalise those found with them.
"If you already have tinted windows and are not sure if they are illegal, I would advise you to contact us so we can check them out."
Tint regulations only apply to the front windscreen and front windows.