Idaho Code § 49-637: Driving on highways laned for traffic.

Whenever any highway has been divided into two (2) or more clearly marked lanes for traffic the following, in addition to all else, shall apply:

(1) A vehicle shall be driven as nearly as practicable entirely within a single lane and shall not be moved from that lane until the driver has first ascertained that the movement can be made with safety.

(2) Upon a highway which is divided into three (3) lanes and provides for two-way movement of traffic, a vehicle shall not be driven in the center lane except when overtaking and passing another vehicle traveling in the same direction when the center lane is clear of traffic within a safe distance, or in preparation for making a left-turn or where the center lane is at the time allocated exclusively to traffic moving in the same direction that the vehicle is proceeding and the allocation is designated by a traffic-control device.

(3) Traffic-control devices may be erected directing specified traffic to use a designated lane, or designate those lanes to be used by traffic moving in a particular direction, regardless of the center of the highway and drivers of vehicles shall obey the directions of every device.

(4) Traffic-control devices may be installed prohibiting the changing of lanes on sections of highways and drivers of vehicles shall obey the directions of every device.