Quote Quoting That Guy
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Well, whoever the contractor/subcontractor was, or the local agency (city's department of public works or whatever it is called in your neck of the woods) performing the work or in charge of the project, should have a records of how the signs were laid out initially, how they were changed, and why.

Did the officer indicate art all what was improper about your turn in particular (never mind the 17 other drivers).... Obviously, you should keep in mind that an "improper turn" could mean a lot more than simply following or not following a left turn arrow
You are correct, however from personal experience with INDOT on a similar issue, state agencies and contractors will do everything in their power to be evasive and non cooperative, when it comes to providing proof of your innocence in a traffic matter. I can't believe those agencies in CA would be of any more assistance. They are very fast to fix their screwups after you point them out though.