Chains are just that: chains that fit over a car's tires, giving much better traction on snow and ice, but requiring drivers to go much slower than normal. That's OK, because when the chains are needed, conditions are really bad.
Drivers should make sure they have the right-size chains for the tires on their vehicle, and put them in the trunk. Also put a blanket or pad in the trunk, to kneel or lie on when installing the chains. Have a flashlight, and a pair of gloves, as well. Practice putting on chains in dry conditions, before having to do it in the cold, snow, and dark.
Chains give the best traction in snow, but are cumbersome, clunky, and can be difficult to put on. They are required on some vehicles on some roads that pass over mountain passes, for example some passes in the Sierra Nevada mountains of California.
In other places, states require chains, other traction devices, or snow tires on the drive axle of all vehicles except four-wheel-drive or all-wheel-drive vehicles.
Where chains are required, drivers must stop and put them on. In some places, drivers can find professional chain installers in areas where chains are required.
The speed limit when chains are required is 25 to 30 mph.
Drivers who choose the put on chains themselves should pull off the road completely, so they are not blocking traffic, or are in danger of being hit. They should also pull off the road completely when removing chains.