I also agree that having the cell phone out is the best response... it changes the dynamic from "big guy in a truck anonymously asserting his dominance" to "what I'm doing is going public."

*sometimes* the right verbal response can relieve the tension but boy, you'd have to have darned good intuition or dumb luck to assess the whole psychology of the situation (which of course you would do without 'profiling,' eh?) and come up with the properly disarming response.