When air meets ground: Modeling the congestion externalities of urban air mobility in integrated air-ground transportation networks

This study develops an integrated modeling framework to analyze the network equilibrium and congestion externalities of Urban Air Mobility (UAM) within a low-altitude air-ground transportation system. The model couples a ground transportation network with a low-altitude airspace network via vertiports, incorporating elastic traveler demand with cross-price effects and employing Macroscopic Fundamental Diagrams to capture traffic dynamics in both layers. We investigate how system controls (perimeter boundary and vertiport operations), UAM network design, and pricing strategies influence overall system performance. Analytical findings reveal significant paradoxes: well-intentioned perimeter and vertiport controls can exacerbate congestion (control paradox), airspace capacity expansion may worsen system-wide conditions (capacity paradox), and fare adjustments can produce counterproductive demand shifts (pricing paradox). These paradoxes are fundamentally driven by the interplay between demand elasticities and the coupling strength between air and ground networks. Numerical experiments based on Hong Kong Island demonstrate that the impact of UAM is highly sensitive to network design: while UAM services with very few routes have limited system-wide effects, a complex, highly connected network acts as a powerful network reconfiguration tool, redistributing demand and fostering a more polycentric and resilient urban structure. Sensitivity analyses further show that the control paradox is amplified by low own-price sensitivity and high cross-price sensitivity for ground controls, whereas airspace and vertiport controls exhibit opposite patterns. The study underscores that the primary value of UAM lies not merely in point-to-point speed but in its potential as a strategic lever for reshaping city-wide mobility. Our framework provides critical insights for policymakers and operators to navigate these complexities and design integrated management strategies that harness the benefits of UAM while mitigating unintended congestion spillovers.