It is perfectly legal to flip off anyone inside your vehicle. That's been taken to court many times and is considered a 1st amendment right.

However, within a courtroom, at the judge's discretion, he/she may chose to put you in contempt of court. Technically there's a problem with that, but I doubt any civil libertarians will take up that challenge for there must be decorum.

Don't think the 1st Amendment protects you anymore. You see the problem? While you have the right to free speech, there's a cost to it as well.