Road material stock and circularity potential in growing cities: the case of Singapore

Abstract In dense and land-constrained cities, road infrastructure systems may transition from expansion-led growth towards a greater reliance on maintenance, with implications for material demand and circularity. Using Singapore as an illustrative case, this study applies a bottom-up material flow analysis to quantify material stocks and flows in the urban road network from 1990 to 2022. Total road material stock increased from approximately 37.2 Mt in 1990 to 50.6 Mt in 2022. Granite aggregate accounted for the largest share of this stock, with smaller contributions from bitumen and steel slag aggregate. Although net stock growth was primarily associated with recorded road-network expansion, maintenance generated a persistent annual material demand through wearing course renewal. The cohort-based maintenance model estimates that 615 lane-km required resurfacing in 2022, generating 0.214 Mt of removed asphalt. Under the baseline assumption, 98% of this material was recovered as reclaimed asphalt pavement (RAP). However, the use of RAP was constrained by the maximum incorporation limits applied to wearing- and base-course asphalt demand, indicating that high recovery does not necessarily result in high internal reuse. Road material stock per capita is approximately 9 t/cap in 2022, reflecting Singapore’s compact urban form and intensive use of shared infrastructure. Material efficiency declined from approximately 460 to 423 vehicle-kilometres per tonne between 2012 and 2022. The findings offer transferable insights for other high-density cities facing limited scope for network expansion, underscoring the need to shift policy attention from supporting infrastructure growth towards extending asset lifetimes, optimising renewal cycles, and improving material circularity in existing road networks.