I run with my dog, mostly off lead around fields and farm land. We rarely encounter anyone else, if we do see walkers / dog walkers he'll probably wander towards them before running away (he's a bit of a coward!) but I know if we see another runner I need to put his lead on - there's just something about runners that excites him and he'll want to go with them! I know he means no harm but I don't want to be one of those dog owners who's shouting 'he won't touch you, he's just being friendly'!
One time, near the end of our run I had stopped (to pick up his poo) and I didn't see an old man running past us - Diesel decided to follow; to be fair he didn't jump or bark, he didn't even run, just trotted along side him, about 2 meters to the side, until I noticed and called on him to wait. Diesel waited and the old man stopped (he was only about 6 - 7 meters away from me). I shouted an apology to the old man who asked me 'does he bite' when I said no he said 'well he doesn't have a sign around his neck saying he doesn't bite' I really didn't know how to respond to that.
I honestly can see both sides of the dogs and runners discussion: I've had some dodgy doggy encounters (one farm house I won't pass because the dog comes running out looking and sounding like he wants to kill me) but an aggressive, out of control dog is very different from a curious dog who's owner's in view. Most dogs I encounter fall into the latter category and unsully I find if I slow down or stop they're no longer iterested.
My scariest encounters have been with horses, now they are dangerous, scary beasts ...