This is from Cal Trans.
Mountain Travel in Winter Weather
Winter weather is expected this weekend in the San Bernardino and San Jacinto mountains. That means it’s time to start carrying chains when traveling in mountain areas. The different levels of chain control are described below.
Snow chains can be purchased at discount, sporting goods, and auto parts
stores. Remember, mountain weather can change dramatically on very short notice.
CHAIN CONTROL REQUIREMENT LEVELS:
R-1: Chains are required on all commercial vehicles (trucks or buses). All other vehicles (cars, pick-ups, vans, etc.) must have either snow tread tires or chains on the drive axle.
R-2: Chains are required on all vehicles except four-wheel drives with snow tread tires. Four-wheel drive vehicles must carry chains in the vehicle.
R-3: Chains required - ALL VEHICLES - no exceptions.