Idaho Code § 49-940: Mirrors.

(1) Every motor vehicle shall be equipped with a mirror so located as to reflect to the operator a view of the highway for a distance of at least two hundred (200) feet to the rear of the vehicle.

(2) When a motor vehicle is so loaded or constructed, or is towing a vehicle or trailer which is so loaded or constructed as to obstruct the driver’s view straight to the rear, then the motor vehicle shall be equipped with a mirror on the left side and a mirror on the right side so located as to reflect to the operator a view of the highway for a distance of at least two hundred (200) feet to the rear of the vehicle.

(3) When an operator of a motor vehicle is transporting under special permit authority an oversize load which makes mirrors impractical devices for reflecting to the operator a view of the highway to the rear, a following escort vehicle equipped with proper mirrors meeting the requirements herein may be substituted for the required mirrors on the hauling motor vehicle. The escort vehicle must be a car or light truck and it must be equipped with an oversize load sign, flashing or rotating lights, and a two (2) way radio which provides full-time communication with the escorted vehicle.