• W: No Restrictions - Watch for snow on pavement.
• R-1M: Chains are required on single-axle drive vehicles with trailers (commonly referred to as R1 Modified).
• 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 on all four wheels and provided that tire traction devices for at least one set of drive wheels are carried in or upon the vehicle.
• R-3: Chains required - ALL VEHICLES- no exceptions.
• Vehicles Towing Trailers: For chain control levels R-1M, R-1, R-2, and R-3, the following applies: All vehicles, including four wheel drive vehicles, that are towing trailers must have chains on one drive axle. Trailers equipped with brakes must have chains on one axle.
• C: Road Closed
• T: Truck Hold - During major storms when traffic flow is heavy, Caltrans may hold tractor-trailer combinations at specific points below the snow line.
• TS: Truck Screening - All semi-trucks will be checked to make sure they have a full set of tire chains before being allowed to travel into areas where chains will be required.
• VS: Vehicle Screening - All vehicles will be checked to make sure they have a full set of tire chains before being allowed to travel into areas where chains will be required.
• MT: Metering Traffic - Implemented when necessary to control the number of vehicles that feed into the snow area. This congestion control procedure gives Caltrans a better chance of keeping the freeway open.

R-1 and R-2 are the most common chain controls. The highway will often be closed before an R-3 control is imposed.