In many of the most densely populated and dynamic economic regions around the world, passengers have access to multiple airports in their neighborhood. Beijing, Hong Kong and the Greater Bay Area, London, New York and Tokyo can serve as prominent examples. Due to geographic accessibility and facility characteristics, some airports within the same multiple-airport region (MAR) may have unused capacity whereas others experience severe congestion. Policy makers in MARs, therefore, face a common challenge: managing the allocation of air traffic among airports. Perimeter rules set upper limits on the distances of nonstop flights operated from specific airports within MARs. Traffic distribution rules such as perimeter rules have been used in the United States and Japan. In this study, we distinguish between misallocation and quantity effects. We demonstrate that in the absence of regulation (laissez faire) a combination of a high demand combined with low capacity, as would be the case for LaGuardia as well as Reagan National in the US, can produce substantial welfare losses due to misallocation. This indicates that there can be benefits from centralized policies in the form of perimeter rules and that the introduction or repeal of such rules requires a careful assessment of the effects on the MAR as a whole.