DOT regulated companies are required to have a written drug & alcohol testing program as stipulated by the mode of transportation in which they are regulated. This generally equates to a Drug Free Workplace policy for the DOT employees.
For non-DOT employees, a separate Drug Free Workplace policy would be required. It could also include non-DOT drug testing policies that are in the DOT written drug & alcohol testing program. For non-DOT regulated companies, the employer elects to have a Drug Free Workplace program which starts with the substance abuse policy.