Very interesting.
On the traffic issue...My little brother was hit by a car in Toronto. She drove away as quickly as possible without even checking to see if he was alright. (He was, luckily.) So I wouldn't say Canadian city traffic is any nicer than American city traffic.
I've never heard a polite clerk in the south say "Uh-huh" in response to "Thank you." At least around here, polite people say "please" and "thank you" and "Yes please" and "no ma'am." Anyone who does otherwise is thought at least somewhat rude. I've heard it's different up north, but that's the way we do things here.
From the posts so far, I really don't see anything all that different about Canadian and American culture. I honestly don't think Americans are so god-awful rude and agressive as some people seem to think we are.