Good points all and I've found that many cyclists out for a road ride forget how easy it is for a driver to not see them in shadows, so it's really important to make sure you're visible...some of the mid to darker jersey colors get lost in the shaded areas of the road. Being a cyclist, I think that I have a heightened sensitivity to spot riders when I'm behind the wheel, but have found that many folks just ride along oblivious to others and even I don't see them until I'm almost right behind them. Drivers need advance notice that you're there, so they can slow down, pass carefully or whatever is needed to keep everyone safe.
Another reeeeeeally important thing to remember is to ALWAYS signal when you're slowing down, turning, etc. Many drivers may not know what each hand signal means, but it catches their attention and reflects well on all of us out there, improving cyclists reputation. If you ride steady and seem predictable (not weaving or pulling out into the traffic lane without signalling, etc), riding along, and being aware, you can prevent lots by seeing what's going on around you and always being ready to react defensively. Since I bike commute alot, I wear the bright colored clothing, have lights and reflection strips and make sure I do what I can for drivers to notice and not come near me!
Re: your backpack and aching back - I agree and found that I didn't like to have anything on my back. Got a rack and bigger gear bag for my commuter bike. Another option to a backpack if you really find you need a bigger bag to hold your purse is a messenger type bag...the cross strap in front doesn't restrict as much as backpack straps and there are several different sizes...mine's by Timbuk2 and they're in the Bay Area (CA). Happy trails!