trucks come in many different sizes, creating the need for a truck classification system. truck drivers are required to have a commercial driver's license (cdl) to operate a cmv carrying more than 16 passengers, carrying a certain amount of hazardous materials, or weighing in excess of . acquiring a cdl requires a skills test (driving test), and knowledge test (written test) covering the unique handling qualities of driving a large, heavily loaded 18-wheeler (e.g., backing maneuvers), and the mechanical systems required to operate such a vehicle (e.g., air brakes and vehicle inspection procedures). it costs thousands of dollars to attend a cdl school. one ata survey found that 86% of truckload carriers had a formal relationship with a noncarrier cdl school; 88% of those had a relationship with a private, for-profit cdl school. critics cite these relationships as evidence that cdl schools are driver mills for trucking companies.