One of the cheapest but most popular methods of customising cars these days is to apply tints to the windows. The procedure itself is hardly new - tints to reduce sunlight glare have been standard equipment for a very long time, and some manufacturers are, partly for security reasons, producing cars with remarkably dark glass in the rear side windows.
None of them supplies that kind of glass in the front side windows or the windscreen because there is legislation to prevent them from doing so. The regulations are reasonably straight forward and would probably have no impact on the general public except for the fact that they can be breached by aftermarket tints.
The responsibility for enforcing the rules among car owners in this country now lies with the Vehicle and Operator Services Agency (VOSA), which was created on 1 April 2003. Full details of the organisation can be found online at www.vosa.gov.uk - the short story is that VOSA looks after things like testing schemes (including the MOT test), licence applications for bus and lorry drivers, advice to commercial operators, accident investigation and so on.
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.
This is, of course, a safety issue. The darker a window, the less chance there is of the driver being able to see pedestrians and other motorists, and take any avoiding action that may be required. On the other hand, darkened windows do make a car look quite dramatic, so it's easy to see their appeal amongst cruisers.