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. Multirunway analytical capacity models also provide good approximate estimates of true capacity in cases involving three or more active runways, as long as the runway configurations can be “decomposed” into semi-independent parts, each consisting of one or two runways. Such models have proved extremely valuable in airport planning, as well as in assessing the impacts of proposed procedural or technological changes on airport capacity.