No person shall drive a motor vehicle at such a slow speed as to impede or block the normal and reasonable movement of traffic except when reduced speed is necessary for safe operation or in compliance with law. Peace officers are hereby authorized to enforce this provision by directions to drivers, and in the event of apparent wilful disobedience to this provision and refusal to comply with direction of an officer in accordance herewith the continued slow operation by a driver shall be unlawful. (2008 Code § 17-294)