I would think that definition would be vision that can't be corrected better than 20/200 or 20/400 or whatever, not just being blind as a bat without one's glasses.

Just in general, I find that poorer vision, as when riding in the dark, is cause to slow down, but not to stop. I know a couple of people that qualify as legally blind. I expect both could ride if they cared to, though they would probably be doing 8 mph and not 20 mph. They can walk around okay and stuff.