Updated: Ask and ye shall receive (link courtesy of a commenter - thanks Chris!)
Its seems like the car horn has basically become an instrument of anger. I don't think you can honk it without seeming mad. Even a tap is a sign of impatience, not communication (think about whenever you're on the receiving end).
I guess CB radios were an attempt to create transient communities for commuters and the like - and cel phones have taken their place for fixed communities, but wierd to me that there's no good way to communicate with the folks around you when driving... maybe this will be one of the emergent expressions of location based services?
In of itself that's surprising - you'd think geography and technology would be a great(er) fit; look at the success of GPS. Like wireless computing, once you do it, you'd can't go back. So why is it so hard finding a behaviour pattern for location enabled computing that's more than a techno-fad?
Important factors (I think): ease of setup, discovery, of course, but more than that - what's the new behaviour eco-system this enables?