Whenever any roadway has been divided into two or more clearly marked lanes for traffic, the following rules, in addition to all others consistent herewith, shall apply.
(1) A vehicle shall be driven as nearly as practicable entirely within a single lane and shall not be moved from such lane until the driver has first ascertained that such movement can be made with safety.
(2) The department may erect signs directing slow moving traffic to use a designated lane or designating those lanes to be used by traffic moving in a particular direction, and drivers of vehicles shall obey the directions of such signs.