Idaho Code § 49-655: Minimum speed regulation.

No person shall drive a motor vehicle at such a slow speed as to impede the normal and reasonable movement of traffic except when reduced speed is necessary for safe operation or in compliance with the law. Notwithstanding any minimum speed that may be authorized and posted pursuant to this section, no person shall operate a vehicle in the extreme left-hand lane of a controlled-access highway for a period of time that impedes the flow of other traffic traveling at a lawful rate of speed.