Road Vehicles (Construction & Use) Regulations 1986 as amended specify the minimum levels of light that must pass through the windscreen and front side windows. The limits are: 

Motor Vehicles first used before 1 April 1985: The windscreen and front side windows must allow at least 70% of light to be transmitted through them. 
Motor Vehicles first used on or after 1 April 1985: The light transmitted through the windscreen must be at least 75%. The front side windows must allow at least 70% of light to be transmitted through them. If the glass is tinted to a point whereby it lets through less light, then the vehicle does not meet legal requirements.

The windscreen and the front side windows to either side of the drivers’ head.

It restricts the driver’s vision, especially in dark conditions. This may prevent drivers from seeing other road users or pedestrians. It also prevents other road users and pedestrians from confirming through eye contact that they have been seen.

The purpose of the law is to ensure the drivers’ ability to see the road is not excessively restricted by glass tint.

If you are the driver:
- You must not drive a vehicle on the road with the windscreen or front side windows excessively tinted. You may also invalidate your insurance with this modification, particularly as the vehicle is likely to be illegal.

If you are a tinting company: 
- You must not modify, or offer to supply, a part that when fitted to a vehicle means that it does not comply with Construction & Use Regulations.

If you’re selling a vehicle with extra tinting applied to the windscreen or front side windows:
- The vehicle may now have glass that is darker than permitted by Construction & Use Regulations, in which case the vehicle should not be sold.

Excessively tinted glass is seen as a serious issue but one which currently affects only a small number of the 24 million vehicles tested annually. To include this item in the MOT test would require all 18,000 garages to incur expenditure on special test equipment and the time taken to carry out an MOT would increase. The MOT fee would have to be raised to cover the extra time and investment. This extra cost would affect all motorists - all for a small number of vehicles. With the current levels of offending, roadside enforcement is a better route as it targets the offenders while minimising the cost and inconvenience to compliant road users.

VOSA staff throughout the country have been issued with testing equipment. If the equipment is not available, a subjective assessment will be carried out. If the vehicle is considered dangerous to drive then an immediate prohibition may be issued.