A written policy is required for employees who are required to perform "Safety-sensitive functions", defined as any driver who requires a Commercial Driver's License to perform his/her job.  This includes Class A, B, and any endorsements such as Hazardous Materials, Air Brakes, etcetera.  This policy also addresses pre-employment, random, reasonable suspicion, and post-accident drug-alcohol testing.

Law:

Code of Federal Regulations, Title 49, Part 382