Just like some of you, I wear contacts for distance but need readers for close-up. So I've been having more trouble reading my Garmin when riding, and I imagine trying to find/dig out goatheads and such from my tires would be a challenge too. I just found & ordered sunglasses with built-in magnification from this place:
http://www.dualeyewear.com/ Sort of like bifocals, but without any correction in the main viewing area. They aren't terribly expensive at $50 a pair.
I just received my order today and like the way they look! I can read my watch, my phone, etc., with these on! I only wish they had adjustable nose pads so they wouldn't rest on my cheeks. Haven't had a chance yet to try them out riding, so will report back in a few weeks!