Your sunglasses might make you look cooler than an ice box, but are they OK for driving? Are they a boon or burden for safe drivers? Tim Shallcross, Head of Technical Policy and Advice at the Institute of Advanced Motorists' RoadSmart division, has put together a set of tips outlining the key information that all drivers should be aware of.
There is actually an EU standard for sunglasses, to ensure the frames are reasonably strong and sweat resistant and that the lenses are shatterproof, scratch resistant and give good protection against harmful ultra violet light.
Sunglasses which show the CE mark (and any sold within the EU should have one) should come up to the standards, but they are also graded into five categories - 0 to 4 - to show how much visible light they let through.
Category 0 specs allow 80% to 100% of visible light through and are fine for driving at any time. Categories 1, 2 and 3 are progressively darker tints, shielding against brighter levels of sunshine and, while they are all fine for daylight driving, they should not be worn for driving at night.
The darkest is Category 4, which lets just 3% to 8% of the light through. They should not be used for driving at any time, and must be labelled as such. Sunglasses on sale in places such as chemists or opticians are marked with the category - usually on the arm.
However, online shoppers should beware. A look through several websites revealed that very few sellers display the tint category or any symbol, so there is no way of telling if the glasses are suitable for driving until they arrive.
Photochromic glasses aren't marked but should be fine for most cars, because they respond to ultraviolet light to darken. Car windows block UV light, so the glasses will stay more or less clear.
Watch out if you drive a convertible though, because once the roof's down the glasses will darken (although at their darkest they still allow about 20% of light through, putting them around Category 2 or 3). Just be aware they will stay dark for quite a while if you drive into a tunnel or underpass or a long stretch of shadows.