Disclaimer: I am not a lawyer, and this is not legal advice.
Bicycles are defined as “devices” instead of vehicles under California law in the CVC 231:
231. A bicycle is a device upon which any person may ride, propelled exclusively by human power through a belt, chain, or gears, and having one or more wheels. Persons riding bicycles are subject to the provisions of this code specified in Sections 21200 and 21200.5.
So, does that make me a devicular cyclist in California?
“No Vehicles Sign” photo by Marcus Quigmire CC BY 2.0 license.
That’s a case where if you did get a ticket, it would likely hold whether or not the law defines a bicycle as a vehicle. One of those laws that’s not actually a law but gets prosecuted that way.
If it’s private property, then what the sign says doesn’t matter too much because they can always tell you to leave whether you are holding a vehicle, device, vehicular device, or a random assortment of bicycle parts (that’s what a bike is called when you are flying and don’t want to pay $200).
Here in California, these “No Unauthorized Vehicles” signs are often posted at service or fire roads which are otherwise open to walkers and cyclists. If no bikes are allowed, a separate “No Bikes” sign will be posted.
It’s simple – in this case, a bicycle is an authorized vehicle. Non-issue.
So if the “no bikes” sign is posted “reasonably” consistently around the state, then that could potentially support a cyclist who was ticketed/cited after moving beyond a “no unauthorized vehicle” sign that was not accompanied by a “no bikes” sign. (You might have to read that twice)