Are police officers in California required by law to attempt to
prevent infractions of the law before they happen where possible?  Not
having a moving violation ticket for 10 years, recently I mistakenly
turned left at a no left turn intersection while a police car was
right next to me. I had my blinker on, waited my turn for traffic in
the opposite direction to abate (about a minute and a half), then
turned. The officer simply laid in wait in the lane next to me, then
swooped in and gave me a moving violation ticket. With over a minute
to act, knowing that I was going to mistakenly turn left, he could
have easily prevented me from doing so by the use of his siren, bull
horn, or simply rolling down his window and saying 'hey buddy, you
can't turn here'. In my opinion he didn't fulfill his
roll/responsibility to 'serve and protect' by enforcing the law before
the infraction occurred (I mean, there must be a reason this
intersection is no left turn anytime rather than just 4-6 PM like the
rest of the intersections on the street), instead he simply let it
happen so he could be punitive by writing a ticket. Can he do that? Is
there anything in penal, civil, criminal, or municipal law that would
prove to a traffic court judge that the officer didn't do the job the
law requires of him by preventing me from entering into the infraction
in the first place?  Violated my rights?  Something else?  I found a
speech by former US Attorney General John Ashcroft where he makes it
clear that as a matter of public policy, law enforcement should seek
to prevent infarctions of the law before they happen (speaking about
St. Louis election fraud in this case), however, I haven't been able
to successfully review the legal codes and precedents to make a
stronger case.  Please site specific codes and/or legal
decisions/precedents that I can print out and take to court with me. 
Thank you in advance for any help with this, hopefully you can see why
this one bothers me so much! -Phil

A law enforcement officer is sworn to enforce the law. Serving and
protecting is indeed part of his obligations but it primarily
responbility as "A LAW ENFORCEMENT OFFICER" is to "ENFORCE THE LAW".
It is YOUR obligation to know the law and to abide by it. By taking
punitive action against you the officer IS preventing it from
happening and it also setting an example for others. By doing what he
did he kept true to his oath:

OATH OF OFFICE

         I ............................ do solemnly swear (or affirm)
that I will support, protect, and defend the Constitution of the
United States, and the Constitution and Government of the State of
...................., against all enemies, whether domestic or
foreign, and that I will bear true faith, allegiance, and loyalty to
the same, any ordinance, resolution, or law of any state
notwithstanding, and that I will well and faithfully perform all
duties of the office of (Police Officer, etc.) on which I am about to
enter; (if an oath) so help me God, (if an affirmation) under
penalties of perjury.

Personally I think you were caught red-handed and it burns you up. I
would not have hesitated to post this as the one and only true answer
under normal circumstances but then I decided the risk that you might
turn on the messenger was just too great.

tutuzdad-ga

You asked "can he do that?", i.e, can the police officer write you a
ticket after watching you make a wrong turn, when he could have
prevented you from making that left turn. Yes he can and I don't think
there is any case laws that can help you because traffic laws are
strict liability offenses. Your best bet is to challenge the citation
in traffic court. Argue to the judge that the subject intersection is
a Traffice Trap, a location used to entrap unsuspecting motorists for
the primary purpose of raising revenue not improving safety. Bring
good pictures and drawings of the location. I think the facts will
support your argument. If you are mad enough, bring this to the
attention of  public officials and lobby to have the law changed. Good
luck.