what's snow chains?

 Snow chains, or tire chains, are devices which are affixed to the tires of vehicles to provide superior traction when driving through snow and ice.

Snow chains are attached to the drive wheels of a vehicle. Chains are usually sold in pairs and must be purchased to match a particular tire size (tire diameter and tread width). Driving with chains will reduce fuel efficiency and can limit the speed of the automobile to approximately 50 km/h (30 mph).

In snowy conditions, transportation authorities may require snow chains to be fitted on vehicles. This can apply to all vehicles, or those that lack certain motive combinations such as four-wheel drive and/or specific classifications of tires. Local requirements may be enforced at checkpoints. Snow chains should always be fitted to one or more drive axles of a vehicle, with varying requirements for dual-tire or multi-driven-axle vehicles that range from 'one pair of tires on a driven axle' to 'all tires on all driven axles' being required to have snow chains

fitted when conditions and/or signage require it.