Extensions to cases involving two or more simultaneously operating runways were also developed at an early stage—see, e.g., Swedish (1981). The complexity of multirunway models depends greatly on the extent to which operations on different runways interact because of air traffic control separation requirements. For example, in the case of a pair of intersecting runways, the location of the intersection relative to the points where takeoffs are initiated or where landing aircraft touch down greatly affects the combined capacity of the two runways. Similarly, in the case of two parallel runways, the capacity depends on the distance between the centerlines of the runways. Approximate capacity analyses for airports with two parallel or intersecting runways are quite straightforward.