Here's a bit from the mailing list discussion, which originally came from ACTC's newsletter as I understand it:
The good news is: A cyclist won’t be cited for bicycling on a
private road.
Note: Areas marked “private property” or “no
trespassing” are off-limits; I’m talking specifically about
a “private road.”
Canon Dr/Hidden Dr are in unincorporated Santa Clara
County, so I contacted the SCC Dept of Roads and Highways
for information. The county maintains the first ~3/4 mile of
Canon (from the intersection with Glen Una). The rest of
the road is privately maintained - i.e., not maintained by the
county. This includes the rest of Canon and Hidden; Hidden
ends at Redberry, and the county does maintain Redberry.
Next, I visited the local sheriff’s office:
West Valley Division
1601 S. De Anza Boulevard
Cupertino, CA 95014
Phone: (408) 868-6600
Fax: (408) 868-6641.
I met with deputy Kim Morganthaler. I told him about the
intimidating motorist we’ve been encountering along Canon
Dr, explained that we want to make sure we’re not doing
anything that is wrong, and showed him the pertinent sections
of the California Vehicle Code (see below). He agreed that the
code restricts the general public from driving vehicles on a
private road, but does not restrict the public from biking or
walking on it. If the motorist contacts the sheriff’s office, they
will explain this to him.
Rather than argue with the motorist, my suggestion is to jot
down his license plate number and contact the sheriff’s office.
I didn’t ask if they would follow up in that case, but it’s worth
a try.
Please ride safely and courteously at all times. In closing, I
will share one more observation. There was exactly one notice
affixed to the glass partition at the front desk in the sheriff’s
office - reminding cyclists to set a good example, obey traffic
signals, stop signs, not to ride two abreast, and warning us
that the sheriff’s office is stepping up enforcement. - Pat
CALIFORNIA CODES VEHICLE CODE SECTION 100-680
490. “Private road or driveway” is a way or place in private
ownership and used for vehicular travel by the owner and
those having express or implied permission from the owner but
not by other members of the public.
670. A “vehicle” is a device by which any person or property
may be propelled, moved, or drawn upon a highway, excepting
a device moved exclusively by human power or used exclusively
upon stationary rails or tracks.
Personally, I don't have much occasion to ride on roads marked as "Private", but it is good to know the law and your rights, in case you find yourself on one.