Becoming DOT certified means that a driver is allowed to drive commercial vehicles on the road and that the vehicle he is driving is safe. The Department of Transportation requires that all drivers of commercial vehicles be tested for safe driving practices and that the drivers also undergo physical exams.
In most states, a commercial driver's licence is required to drive public transportation vehicles, semi-trucks and local delivery trucks. Employers hire those with a commercial driver's licence because they know that they have been through the Department of Transportation process of certification via physical exam, which is good for 24 months.