MrS,
With your vision problems especially, you might want to consider using a mirror to help you keep what's behind/around you in your field of vision. You could put one on your helmet (I use the Third Eye brand mirror) or you could get a bar end mirror that plugs into the ends of your drop bars.
Many is the time when I could NOT actually look back safely during a particular tricky road situation but was able to quickly glance at my helmet mirror to see if I was safe behind. It's amazing how a tiny little 7/8 round mirror will give you a panoramic view of the entire road and both road shoulders behind you.
I will say that it takes 2-3 weeks of constant use to get used to a mirror, but once you do, it sort of "disappears" and you dont notice it blocking your front side vision anymore- you just sort of see through it. My mirror has helped me avoid MANY a hairy situation on my bike in traffic. I would not ride without one now.