Understanding the scaling of transport energy use with operational density

Given the substantial contribution of transport operations to global energy demand, enhancing their energy efficiency is crucial for sustainable urban mobility. This study investigates whether intensifying the use of fixed transport networks, termed operational densification, reduces energy consumption. Grounded in economic theory, we develop a novel causal model to estimate the energy impacts of densification across two major commuting modes: urban rail transit (metro) and private car travel. Using a unique panel dataset of 27 metro operations worldwide, we find that a 10% increase in passenger-kilometres travelled on a fixed network reduces energy use per passenger-kilometre by 3.45%. These gains surpass what kinetic energy principles alone predict, as fixed energy inputs such as infrastructure and maintenance are distributed across increased usage. In contrast, analysis of the Millennium Cities Database reveals no significant energy savings from densification in private car travel, likely due to limited shared infrastructure or operational scale economies.