The work that is presented in this paper is part of an ongoing study on the relationship between airspace structure and capacity. The present paper investigates the degree of structuring needed to maximize capacity for decentralized en route airspace. To this end, four decentralized en route airspace concepts, which vary in terms of the number of constrained degrees of freedom, were compared using fast-time simulations, for both nominal and non-nominal conditions. The airspace structure–capacity relationship was studied from the effect of multiple traffic demand densities on airspace metrics. The results indicated that structuring methods that overconstrained the horizontal path of aircraft reduced capacity because traffic demand displays no predominant patterns in the horizontal dimension for decentralization. The results also showed that capacity was maximized when a vertical segmentation of airspace was used to separate traffic with different travel directions at different flight levels. This mode of structuring improved performance over completely unstructured airspace by reducing relative velocities between aircraft cruising at the same altitude, while allowing direct horizontal routes.