OK, I apologize for misinterpretting what you were saying. They could charge you with either section, but either way the turn out sign only applies to the five car following rule, you can certainly argue that. Are we talkign about MM23 in Mendocino county? Which direction were you going?
The other thing to note is whether there was a R4-12 sign at the beginning of the stretch with turnouts. I'm not seeing it going eastbound out of Ft Bragg. The MUTCD says SHALL there (that's imperative).
I was going westbound (leaving from Ft Bragg), yes Mendocino county was what appeared to be written on citation.
There appears according to MUTCD that there should be a sign warning you about pulling over with 5 people behind you. I don't see it on Google StreetView but that doesn't mean it's not there. SOME of the turnouts are marked (I think the one you are accused of missing is), but I think you do have a compelling argument that if there weren't five cars behind you the SLOWER CARS USE TURNOUTS doesn't have validity. Absent the R12 sign (you might give a quick pass from Ft Bragg up to the first turn out again). For some reason I had you way at the other end of HWY 20.
You will need to articulate your argument properly if you want to prevail. If it was me, I would be sure to make the following points:
1. You were driving on 20 and exactly 3 cars were behind you.
2. There were no R4-12 signs posted (be sure to describe the sign) (I've never seen those on 20... were they there?)
3. You were cited for disobeying a R4-14 under 21461
4. Quote the applicable section of 2B in the MUTCD and explain that R4-14s are required to be used in conjunction with R4-12s. I believe the CAMUTCD explains the reasoning for this. You can reiterate it.
5. As such the sign was not regulatory in nature, per CAMUTCD, and you were not in violation of 21461
6. Furthermore, You were not cited as violating 21656, the more appropriate code.
7. Quote 21656
8. State that the legislature's intent was made clear with this statute, that use of turnouts are mandatory when there are 5 or more cars behind you.
9. Argue that the officer should not circumvent this by citing 21461.
Given the code he cited you under and the MUTCD evidence, I believe this argument to be compelling and that TBWD is sufficient.