On Sun, 24 Jul 2016 14:35:29 -0000 (UTC), Henry Jones wrote:

I'm thinking of putting down a big "stencil" on the road of something like:
"PRIVATE DRIVE | NO PULIC ACCESS"

According to this document:
http://www.indiopd.org/pdf/Trespassing_Requirements.pdf

Legally, apparently, California no-trespassing signs must say the explicit
words "Trespassing/Loitering Forbidden by Law", and the letters must be at
least 2 inches in height, and the sign must be at least 1-foot square
posted at no less frequency than 3 per mile and at the entrance and exit to
all entrances to any property that wants to enforce the no-trespassing
rule.

A suggested legally accepted text is:
PRIVATE PROPERTY
TRESPASSING/LOITERING FORBIDDEN BY LAW
VIOLATORS WILL BE PROSECUTED
CALIFORNIA PENAL CODE 602

In addition, and only after adding the legally required signage, homeowners
"could" send an official form every six months to the police who can
enforce the rules in their absence (but they will only do so if the
appropriate signage is already in place).