In an interview with Tom Vanderbilt, author of the upcoming book Traffic :

"Then there’s the anonymity in traffic — there’s no one to spread rumors or gossip about you about how bad your behavior was — not to mention the lack of consequences for acting like an idiot. It’s all strikingly similar to the way we act on the internet, in what’s called the “online disinhibition effect.”

We can lurk behind screen names (our car and license plate), say nasty things about people (honk, give the finger), and then sign off and never be heard from again (drive away).

In a nice convergence of this, the website PlateWire allows people to anonymously insult others’ rude and unsafe behavior."

http://freakonomics.blogs.nytimes.com/2008/06/05/hows-my-driving-a-qa-with-the-author-of-traffic/