The Road Vehicles (Construction & Use) Regulations 1986 (as amended) specify the minimum levels of light that must pass through the windscreen and front side windows of a vehicle.
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.
A fixed penalty of £30 can be imposed as a minimum.
It is down to Police discretion as to how they deal with the offence. However, be warned that, whilst the offence of driving with reduced visibility is punishable by a fine only, the police could decide to report you for the more serious offence of careless or even dangerous driving if you drive around without waiting for your de-icer to take effect.
A motorist can be faced with a prosecution for careless driving punishable by between 3 to 9 penalty points and a hefty fine for simply failing to adhere to the Highway Code.