Not the way I see it...
Here is the Vehicle Code definition of an alley:
110. "Alley" is any highway having a roadway not exceeding 25 feet in width which is primarily used for access to the rear or side entrances of abutting property; provided, that the City and County of San Francisco may designate by ordinance or resolution as an "alley" any highway having a roadway not exceeding 25 feet in width.
So as long as it fits within the above definition, then the rules of the road would be equally applicable.
Here is the Vehicle Code definition of a "Highway":
360. "Highway" is a way or place of whatever nature, publicly maintained and open to the use of the public for purposes of vehicular travel. Highway includes street.
So, as long as an alley is open to the use of public for purposes of vehicular travel, then (again) the same rules of the road would apply...
And here is the section you were cited for:
22101.
(a) The Department of Transportation or local authorities in respect to highways under their respective jurisdictions, may cause official traffic control devices to be placed or erected within or adjacent to intersections to regulate or prohibit turning movements at such intersections.
(b) When turning movements are required at an intersection notice of such requirement shall be given by erection of a sign, unless an additional clearly marked traffic lane is provided for the approach to the turning movement, in which event notice as applicable to such additional traffic lane shall be given by any official traffic control device.
(c) When right- or left-hand turns are prohibited at an intersection notice of such prohibition shall be given by erection of a sign.
(d) When official traffic control devices are placed as required in subdivisions (b) or (c), it shall be unlawful for any driver of a vehicle to disobey the directions of such official traffic control devices.
You can check the Federal Manual on Uniform Traffic Control Devices to see whether those particular signs fit the definition of official traffic control devices and then check the California Supplement to that manual to see whether the placement of those signs fits the requirements stated therein.
Keep in mind that there are often times where perfect placement of such signs can not be achieved and that the engineer can deviate slightly from the Standard/Guidance/Options suggested in those manuals. SO the determining factor here would be whether a reasonable person (no pun intended) can see and properly interpret the sign and the purpose for which it is posted.
I am not preview to the conversation you had with the officer at the time of the stop or whether you admitted to seeing and misinterpreting the sign (as you stated above)... Though you lack of familiarity with such sign is not a valid defense.
Sooo.. The only "out" that I can see here is if the officer does not submit his declaration (if you choose to go with the Trial By Declaration option) OR if he does not appear at your trial in court (if you choose the in-court trial option)...
Good luck!

