Whenever any roadway has been divided into two or more clearly marked lanes for traffic, a vehicle shall be driven as nearly as practicable entirely within a single lane and shall not be moved from such lane or across a lane line without giving the right-of-way to vehicles in the lane to be entered, nor until the driver has first ascertained that such movement can be made with safety and he has given the appropriate signal. A violation of this Section is an infraction.