Failure to maintain single lane or FTMSL is a traffic rule violation. A police officer can pull over a driver if the driver's tires cross a lane divider or the edge line. This is a common motor vehicle offense and is often charged in conjunction with other traffic tickets. The laws of most states say “Whenever any roadway has been divided into two or more clearly marked lanes for traffic in one direction, vehicle shall be driven as nearly as practical entirely within a single lane and shall not be moved from the lane until such movement can be made with reasonable safety.”
The following is an example of a State Statute (Alabama) on traffic lanes:
Code of Ala. § 32-5A-88. Multiple lanes.
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) Upon a roadway which is divided into three 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 such center lane is clear of traffic within a safe distance, or in preparation for making a left turn or where such center lane is at the time allocated exclusively to traffic moving in the same direction that the vehicle is proceeding and such allocation is designated by official traffic-control devices.
(3) Official traffic-control devices may be erected directing specified traffic to use a designated lane or designating those lanes to be used by traffic moving in a particular direction regardless of the center of the roadway and drivers of vehicles shall obey the direction of every such device.
(4) Official traffic-control devices may be installed prohibiting the changing of lanes on sections of roadway and drivers of vehicles shall obey the directions of every such device.