I was talking with some folks from Canada at an event I recently attended during the Desire2Learn Fusion 2010 conference in Chicago. One of the things they pointed out was that, in Chicago, you hear constant taxi horns. Now, sitting in my hotel room, I can't seem to block them out.
I think taxi cab drivers have developed a code with which they communicate via their horns. It has nothing to do with the warning "beep" that you and I use, or even the "angry beeeeppp" that you occasionally hear (and fear). No, I think they've taken "tweet"ing literally and let each other know what they think of their passengers, their current destination, what they're doing after work...all manner of things through the subtle shifts in tone and length of the beep. How else do you explain that I hear constant taxi horns outside my hotel room, but have not yet heard or seen a single crash to go along with any of them. Moreover, you hear them even when there are only two w-i-d-e-l-y separated taxis on the same street.
I suppose it could be extremely defensive and safe driving habits, but when was the last time you thought to apply that description to a taxi driver? I think it's safer to assume that it's not as primitive a signaling system as you and I are used to. There's much more than simple emotional content blasting across the streets of Chicago; who needs 140 characters to get your message across when you have a few seconds of car horn.