In the California Sierras, they are more
of a law enforcement formality than anything.
The law here states that AWD vehicles do
not have to chain up even if the road is
posted for chains. OTOH, if you do get
stuck and you can't chain up, you can be
cited.
We carry nice new chains that have only been
mounted once in the warmth of our garage
to check fit and clearance.
Good idea to carry them, this is from CALTRANS website:
Requirement Two (R2): Chains are required on all vehicles except
four wheel drive vehicles with snow tires on all four wheels.
(NOTE: Four wheel drive vehicles must carry traction devices in chain
control areas)
Requirement Three (R3): Chains are required on all vehicles, no
exceptions.
See below (California Vehicle Code p 27459) for definition of
Snow tires.
Security Chain Company, who manufactures traction devices, and runs
the website:
http://www.scc-chain.com/Pages Traction/traction.html
has lots of traction information, along with several types of cable
and link chains that meet the SAE Type "S" requirements.
The limited clearance requirements mandate the "Type "S" chain.
This website is from CALTRANS and designates the three types of chain
controls that may be in effect.
http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm
From the California Vehicle Code:
27459. No person shall operate any motor vehicle, trailer or
semitrailer upon any portion of a highway without tire traction
devices when that portion of the highway is signed for the
requirement of tire traction devices. In any case where a passenger
vehicle or motortruck having an unladen weight of 6,000 pounds or
less may be required by the Department of Transportation or local
authorities to be equipped with tire traction devices, the devices
shall be placed on at least two drive wheels, or the department or
local authorities may provide, in the alternative, that the vehicle
may be equipped with snow-tread tires on at least two drive wheels
when the weather and surface conditions at the time are such that the
stopping, tractive, and cornering abilities of the snow-tread tires
are adequate. The snow-tread tires shall be of a type and design
manufactured for use on snow as a replacement for tire chains or tire
traction devices, shall be in good condition, and shall bear the
marking of M-S, M/S, or other marking indicating that the tire was
manufactured for use on snow, or, in the case of tires purchased
before January 1, 1987, shall either bear the markings or, in the
opinion of the inspecting officer, comply with the tread pattern
requirements of Section 558.
All in all, it looks like it depends on the highway, local
jurisdiction, and the judgement of the CHP/LE officer.