I have "photo gray" bifocal no line plastic lens glasses for the aging eyes in my skull. I have only mild near sightedness, about 20-50, and can really read ok in most cases without reading glasses. However, when I use glasses for distance, it is very hard to see things clearly only a couple of feet away.
The no line bifocals are great for driving, especially at night, as you can see the road, and also read the instruments on the dash board.
However, they are not a great substitute for sun glasses in very bright sun. For one thing, the glass in a vehicle filters out much of the ultra violet that makes the photo grays go dark.
When in very bright sun, I also have a pair of Serengeti sunglasses, prescription, and a pair of prescription sun glasses in the Nikon frame that are dark reddish. The Serengetis are darker at the top than at the bottom, and the lenses are quite large. I got mine mail order over the net, but I cannot remember the source. I understand you can get bifocal, no line Serengetis, but they cost $$$