the fmcsa defines a cmv as a single or combination (truck and trailer) vehicle with a gross weight of or more, or a vehicle used to transport hazardous materials in quantities requiring the vehicle to be marked or placarded under the hazardous materials regulations. the length of time a driver may spend operating a cmv is limited by a set of rules known as the hours of service (hos). these laws are designed to protect the general motoring public by reducing accidents caused by driver fatigue.