Chains-
Here in Colorado semi-trucks have to carry them during Fall & Winter. I've seen them stuck on I-25 at Longmont where it appears flat but where the road climbs out of a river valley and there is a slight grade. On ice, the unchained trucks get stuck. (Heard on the scanner as I was bailing out of work during a blizzard- 5 trucks stuck in the right lane, 5 minutes behind me the left lane got blocked by an unchained semi and the road was closed for the next 12 hours)

Some vehicles cannot accomodate chains on all wheels. The popular Jeep Cherokee (XJ) cannot take them on the front wheels because there isn't adequate clearance between the tires and the tie rod ends. This info is in the owner's manual (as well as being something visible to you if you spend time under your vehicle as I do).