Originally Posted by
lineinthewater
But with video as a witness - can they really ignore the proof?
Yes they can.
Many departments have policies that require that they ignore it -- if a minor moving violation was not personally witnessed by an officer, they can not ticket for it. (More serious things they'll actually investigate -- but if it's just a standard moving violation, no.)
Are they not legally obligated to act on it?
Obligated by whom? And what's the recourse if they don't?
You might find a law (or policy -- doesn't have the strength of a law, but it's pretty good) that requires them to take a report, but I doubt you'll find them obligated to much more than that.