Under 30MPH any glasses will work Ok although you will sometimes get bugs and road grit in your eyes if the wind can flow around the frames, also some eye watering. On the highway the only thing that works well is a visor. Small rocks kicked up by cars and bugs at 60MPH+ really hurt on the face when they hit and can cause serious eye damage at that speed. Unsealed glasses do not block stuff getting in your eyes and can cause a loss of vision due to eye watering from the wind. Also lens that are not shatter proof can cause serious eye damage if you are unlucky enough to crash or get hit in the lens with a rock kicked up by a car or truck.

IMO at anything over 30MPH a vented FF helmet is way more comfortable. I usually ride with a FF helmet and regular sun glasses. In hot weather I raise the visor at stops or at speeds below 20MPH, above that the helmet venting keeps me cool and the visor keeps the wind out of my eyes. In colder weather or at higher speeds nothing else matches the comfort.