Impeding traffic is typically defined as when a driver is not operating their vehicle reasonably and blocks the normal flow of traffic. The laws governing this violation vary by jurisdiction; however, they generally follow a standard of “reasonable operation”. An example of a state law statute is:
The law states that it is unlawful for the driver or operator of any vehicle to operate or stop a vehicle in such a manner as to block or obstruct any roadway to prevent the free use of any roadway for the purpose of travel by other vehicles, either willfully or when such driver or operator is able to avoid doing so by ordinary care.
In a marked area, drivers must maintain forward driving. If you back up on a shoulder or roadway of a highway, you could face penalties. Backing up on a freeway is any backward movement in an area where forward motion is only allowed.