Here are the basics for CA:

Although Caltrans does not post signs with these designations nor use them to announce chain controls to the public, they are used internally within Caltrans and the CHP as a kind of shorthand to describe chain restrictions and may be included in traffic reports disseminated by various news outlets.

There are three primary categories of chain restrictions, as shown below:

R-1: Chains are required on all vehicles except passenger vehicles and light-duty trucks under 6,000 pounds gross weight and equipped with snow tires on at least two drive wheels. Chains must be carried by vehicles using snow tires. All vehicles towing trailers must have chains on one drive axle. Trailers with brakes must have chains on at least one axle.

R-2: Chains are required on all vehicles except four-wheel-drive vehicles under 6,500 pounds gross weight and equipped with snow tires on all four wheels. Chains for one set of drive wheels must be carried by four wheel-drive vehicles using snow tires.

R-3: Chains are required on all vehicles without exception.
R-1 and R-2 are the most common conditions. A highway will often be closed before an R-3 condition is imposed. Some local areas may use variations of these designations. You must follow the directions on the signs posted for chain controls or any instructions given by Caltrans or CHP personnel at chain control check points, even if these are at variance with broadcast road condition reports or information contained herein.

So to quickly answer your Subaru question, no. Just fly right through as long as you have at least M+S tires with 1/2 tread.

But as with all things government, I've found it doesn't work so well in practice when you have merely a 2WD vehicle with M+S rated tires of sufficient tread depth. That's because they don't announce R1 or R2 on the signs. Just "Chains Required." So which is it, guys?

Back in the '90s my mom's Audi wasn't a Quattro. I'd usually zip right through since they all look the same, except when there was Mr. By The Book manning the checkpoint. If I got questioned, I'd ask what the control level was. Answer would always be "chains unless you have 4WD." What's the alternative, be a dick and say you want to talk to a supervisor? I'd always carry some lame-*** traction cables that I could pull over and install just to be done with it.

 

Thanks. I'd seen the chain control signs when touring in the Northwest, and wondered how stringent the controls were.