For urban, the only thing I would really be worried about is drawing attention to yourself. A whistle (Fox40, $4.95), a cellphone, and a flashlight (Target's River Rock two-AAA LED, $9.95) would be about all you'd need for that. I always carry a folding knife too (Spyderco Native, $39 at Walmart), but that's not just for urban walking. I don't even walk around inside my house without carrying the knife!

For a medical emergency or getting hit by a car, you're pretty much dependant on others for help. So just draw attention to yourself.

For a gang of teenagers attacking a lone 50+ year old, about the only thing you could carry to possibly equalize that would be a gun. But you should suspect the kids doing such things might be carrying weapons too, so consider the circumstances before doing something stupid. A man carrying a gun who will still consider giving up his wallet (without drawing the gun) is a smart man indeed. I do not consider my knife as protection against human threats. A younger, stronger assailant might easily turn the tables on you. I would not consider trying to use my knife for defense unless I was convinced my life would be lost were I not to attempt such defense.

For a violent dog threat, my plan would be a strong offense if I were able. Keep your eyes open so you see the threat materializing. Don't wait for the dog to have it's jaws on your throat. For the dog case, I suspect that Spyderco knife would be a great asset. I would have mine out and ready before the dog made it's initial run at me.