Improper lane usage is defined by the state of Illinois as driving a vehicle in any way other than what is practically possible within a single lane of traffic and not moving out of that lane until the driver has determined that the move can be safely made without hampering the operation of other vehicles. This is considered a moving violation punishable by a fine.
DriversRights.com and the Washington State Legislature note that there are different driving mistakes that can constitute an improper lane usage infraction, such as changing lanes frequently without signalling, changing lanes without looking and causing an accident, driving on the left side of the road, driving in the emergency lane and driving in two lanes at once. It is not illegal for a driver to swerve into another lane to avoid an obstacle in the road, such as debris or an animal, however it is illegal for the driver to swerve without signalling and first checking to make sure that the maneuver can be done safely without endangering other drivers. According to Illinois attorney Francis J. Discipio, an improper lane usage violation is often used as a tool for a law enforcement officer to pull over a driver suspected of driving under the influence.