I was walking down the street earlier today, and somebody is walking behind their dog with it off it's leash, and the dog stares at me as I walk past. It then starts growling at me for a few seconds, and I brace myself for it to jump at me, but instead, it runs away from me.

yep.
on a few occasions, only they've been lone dogs.

at this point i usually give up attempting to help them, but with the last one, after seeing him almost run down no less than 6 times as he crossed rather busy roads, i wasn't giving up again.
i followed him down a cul de sac
sat down on the grass and waited to see what he did, by chance, he came near
he came over to cautiously greet my dogs)
then when close enough, i grabbed him by the collar.
got bitten for my trouble, ended up on antibiotics for a week, but once he'd calmed down he was a lovely dog.
ulcerated eyes, so no wonder he was cautious and scared when i did grab him.
poor bugger could barely see.

God that is awful xdow. What did you do with him?

called the dog warden who had already had him reported to them as missing
they put us in touch with his owners and they came straight out to get him
happy ending thankfully, just hope he doesn;t get loose again, he'd come from the other side of the city, how on earth he did that without being run down is a miracle!