You won't go down unless a person hits you really hard which usually means his/her front wheel quick release skewer going into your rear wheel, or their wheel ramming your derailleur pretty hard and tearing it up. Even then you probably won't fall if you don't panic. A good friend of mine used to regularly pull up behind me at the beginning of our group rides and tap my rear wheel to say hi. I'd look back and he'd just have a big grin and a giggle and ask what was up. I've never had a rider who didn't know me sit on extremely close (3" to 6") until we've ridden a while. Less experienced riders usually hang about 3 feet back at a minimum. You can always look back under your armpit to see how far they are, if they're solid, stable, etc... a shadow tell alot about pedal stroke and smoothness as well.
That said Kate, asssuming you're a woman, there are certainly and sadly personal saftey issues that female cyclists have to deal with that men will never know. Guys, treat the women like your sister or your mom... simple respect for their wishes while being courteous and cordial. Tell them what you're doing and where you're going. If you ask if they'd mind a riding partner for a few miles, don't be put off if they don't. The road isn't the place to hit on a woman, but it is a nice place to meet some. Paths cross frequently in cycling communities.
Last edited by puddin' legs; 08-25-05 at 11:33 PM.