Greenham, if you haven't already, instead of looking for prescription lenses/goggles, you should take a peek at the modular helmets like the Shoei Neotec. They have a drop down sun goggle visor when it gets too bright.
Besides the safety concerns with a 3/4 length (your face, your choice), the bigger issue with prescription glasses is that almost none of them have impact resistant lenses rated for things that would hit your eye at highway speed. If you are doing 60 mph and a healthy size bug beast nails itself to your sunglasses or goggles, the lens might not be sufficiently impact resistant. I learned this when I was shopping for prescription sunglasses I could wear in my motocross helmet for dirt bike riding - I rarely exceed 35 mph since I do mostly single track trails.
None of the cheaper options feature impact resistant lenses - you have to pay decent dollars to get progressive lenses with that feature. It is far cheaper and more convenient instead use a face shield (or a modular helmet) that lets you use your current glasses setup. I've ridden with the motocross helmet at speeds up to 65 mph with the prescription wrap-around, and while they help with the wind, it is nothing I'd want to wear for more than a few miles on-road between trails - too much dirt/dust and wind gets by, even with the clip in foam piece.
Of course, if you are wearing a 3/4 length helmet without a face shield.... well I can't help out any more. The new modular helmets, especially Shoe, have come a long way and there really is no excuse not to try them out, if heat is your main concern.
Don't get me wrong. I have the gorgeous 3/4 helmet without shield below and it looks fantastic - on my desktop, not my head.
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