VOSA is itself empowered to stop vehicles for roadside checks in North Wales, Cambridgeshire, Staffordshire, Northumberland and Greater Manchester. Elsewhere it works in conjunction with the police.
The legal position is that the front side windows on all cars must allow 70% of light to pass through them. That figure also applies to the windscreens of cars first used before April 1985; any car first used from then onwards has to let 75% of light through the windscreen.
If the meter registers figures better than those mentioned above during a roadside check, there is no problem. If between 45% and 65% of light gets through, the driver is advised to have the tinting removed, failure to do which might result in a prosecution and a fine of up to £2000.
For between 30% and 45% of light transmission the vehicle is assumed to be unfit for use but not to pose an immediate danger. A delayed prohibition is issued and the owner has up to ten days from the date of issue to remove the tint.
At less than 30% the legal position is that driving the vehicle would involve a risk of injury, and the vehicle cannot be moved under its own power from the issuing of the prohibition notice until the tint has been removed.