I think the correct idea is that you can't simply present your photo and say that because the sign was obstructed, at least from one position as the driver approaches, you are therefore Not Guilty. However, MUTCD or not, you might be able to show that a reasonable and prudent driver might be unaware of the sign and therefore, unknowingly make the prohibited left turn. You might present this defense to two different judges and get two different answers. It seems to me that the tree obstruction makes this well worth the effort at Trial By Written Declaration, sending one or more photos along with your TR205 Form. California Law is very friendly here as you can still go into court later if you get the wrong verdict at TBD.

