Well, as a kid, we had a ton of dangerous dogs in our neighboorhood. Loose dogs are the most dangerous predators around here (well, there's the occasional mountain lion). There's really only three things to do.
1.) Jump over the nearest fence. Chances are are high that if you're in an urban area, you're probably not much further than 20 feet away from a fence. I've hopped into other people's yards before to get away from pissed off dogs.
2.) Hit the dog with a stick or bat. Like I said before, my 'hood had lots of loose dogs around, so I would always keep a bat around when my friends and I were playing In the street. Once a big german shepard came at us, acting all agressive, so I grabbed the bat and whacked it a couple times. It ran off whimpering, i think i might have busted it's ribs.
3.) Carry a knife. If you run out of all other options, or you just want a little protection from dogs and crazy people while you're out running, you should carry a pocketknife. Make sure to stab the dog in it's throat or eyes. Here's the big, light knife I carry around:
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