Whenever any roadway has been divided into two (2) or more clearly marked lanes for traffic, the following rules in addition to all others consistent with this subchapter shall apply:
(1) A vehicle shall be driven as nearly as practical entirely within a single lane and shall not be moved from the lane until the driver has first ascertained that movement can be made with safety; and
(2) Official signs may be erected directing slower-moving traffic to use a designated lane or allocating specified lanes to traffic moving in the same direction, and drivers of vehicles shall obey the directions of every such sign.