Yes that is legal.
That the reporting officer did not observe the incident goes to the weight of his testimony in the trial against you, not its admissibility.
An officer does not have to witness a crime to charge someone with it. Even a ticket. I recommend that you hire a ticket attorney. Their fees are reasonable and they will always save your money.
Macy Jaggers's answer to a legal question on Avvo does not establish an attorney-client relationship. Ms. Jaggers offers everyone a free consultation to discuss their case. Feel free to call her office at 214-365-9800 to make an appointment (phones are answered 24 hours) or visit her website at www.macyjaggers.com for more information about her services and recent victories.
If you decide to have a trial on the matter, it can be harder for an officer to properly answer questions on cross exam if he didn't witness the incident. However, it's not impossible--in NY, an example might be an officer who testifies that skid marks were present after a rear end accident on the highway may be able to successfully sustain a charge of following too close.
Yes it is legal. Whether the testimony of the officer will hold up in court is another story . You need an attorney to desk with this as a guilty verdict can have consequences beyond just the ticket. There will be a civil action with respect to the collision .
Yes, the officer can issue a ticket after the fact. When the report was filed, apparently you told the officer what happened, your side of the story. You told him that you were making a u-turn after a left. That is an admission. It will be used to prove that you were making a u-turn. Whether or not you were making it unsafely is what is at issue. Just because you were making a u-turn and a collision occurred doen't mean you caused it. You need a lawyer.