Varying light conditions and glare can cause drowsiness and reduce concentration. Because the windscreen will take away 40% of the UV risk, this will reduce the effectiveness of photochromic lenses(which darken in sunlight).
The best choice is a high-contrast lens that allows the eye to react naturally to changing light conditions. Polarising lenses will also help as they are specially made to reduce reflective glare off flat road surfaces and relax the eyes.
Also, make sure the sunglasses are in the filter category range of 0-3. A lens carrying a filter category of 4 will be too dark for safe driving. Never wear sunglasses when driving in poor light.
Recommended makes: Revo, Serengeti and any good quality polarising range.