I don't know how this works in america, but from my experience working in a UK opticians certain limits are there at which point the person is classed as having a complex prescription and must legally receive help towards their eyecare costs.
If interested, these limits are a spherical power of over 10 dioptres in either direction, OR a cylindrical power of 5 dioptres in either direction - there's a limit for prism prescription too but I can't remember what that was as I never came across it very often.
If the sight can be fully corrected with glasses then the person is legal to drive with their glasses on, if not then they aren't allowed to drive.
Now obviously this bit is speculation, based on this the glasses in this photo -
https://fbcdn-sphotos-h-a.akamaihd.net/hphotos-ak-ash3/733859_526877524025840_1602000303_n.jpg
From what I can tell, Mr Harrod had a prescription for myopia, but certainly nowhere near the size of a 10 dioptre lens (unless they've been thinned, but they look too thick for what I've seen from even the very upper limit of lens thinning.) He doesn't seem to have any extreme astigmatism needing to be corrected by a high cylindrical power. He appears to be wearing bifocals, so those aren't his reading glasses which would have a lower power if he has myopia.
I'd assume he was legal to drive providing he was wearing those glasses, but his prescription was not high enough to breach those upper limits.