i doubt it's actually illegal. it's a bad idea though and you should never do it or even slow down except to avoid hitting them yourself. the problem is, if you wave someone across the street and they get clocked in the next lane by someone else, you will become a defendant in a civil action for damages under what is called joint and several liability. don't take it personally, by the way - the plaintiff's lawyer has no choice in suing you. if a jury finds you somehow, say, 20 percent responsible and you're not named as a defendant, the lawyer would be sued for malpractice. usually, you'd get dismissed at some point from the case or your insurance carrier will pay a small settlement if it's less than the cost (also accounting for risk) of filing a motion for summary judgment to get you out of the case. but there would be residual issues there such as possibly increased insurance premiums. bottom line - don't do it.
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