It's a conundrum.
I was at the Second Cup drive thru. J was in the back of the car. I placed my order. I drove up to the window. In the distance, a car horn blared.
J: You IDIOT!!
The girl at the window couldn't stop laughing.
As a parent, what do you do? The behaviour is inappropriate. A 3-year old should not call someone an "idiot". You can't laugh; it encourages repeat behaviour. You also can't scold; it's perfectly obvious that he learned this behaviour from you. So you blush, mumble something uninteligible to the Second Cup drive-thru girl, take your order, and drive off into the night.
Road rage is a skill that must be honed at an early age.