A lane assignment system includes a digital-map, a ranging-sensor, and one or more controller-circuits. The digital-map indicates a position of a host-vehicle traveling in a travel-lane on a roadway. The ranging sensor detects a lateral-distance to an other-vehicle traveling on the roadway proximate the host-vehicle. The one or more controller-circuits are in communication with the digital-map and the ranging-sensor. The one or more controller-circuits determine a lateral-variation of the lateral-distance, determine whether the lateral-variation is greater than a dynamic-threshold, determine whether a second-lane exists beyond a first-lane based on the digital-map, determine that the other-vehicle is traveling in the first-lane, and operate the host-vehicle in accordance with the other-vehicle traveling in the first-lane.