[antonye]

Because us mere public can't be trusted to have any slightest hint of common sense and change a dark visor for a clear one when it starts to get dark!

It would be a legal nightmare to define, but would most probably be based around dawn/dusk timings for when one a dark visor becomes illegal. You then get problems with weather (heavy rain clouds can make it very dark) and other problems like long tunnels.

At the moment we have a law suitable for all conditions, which is the safest option.