Correlation or cause? The limitations of population density as an indicator for public transport viability in the context of a rapidly growing developing city

Abstract

Abstract In response to high urbanization rates in developing countries, many city authorities are embarking upon large scale public transport improvement programmes. Supportive land use environments will be necessary to reduce operating subsidies to within sustainable limits for these developing economies. In past decades, the literature on the relationship between land use and public transport viability has been dominated by city population density, which is posited to have a strong causal link. This has led to the recommendation of minimum density thresholds for viability. However, for some rapidly growing cities in developing countries, public transport systems have not attained sustainable subsidy levels despite city population densities significantly above the purported thresholds. The aim of this paper is to investigate the significance of population density as an indicator of public transport viability in rapidly growing developing city contexts generally, and in South African cities in particular. A public transport corridor operating cost model was developed for this purpose. The cost modelling results suggest that the causal link between city population density and public transport viability is only leveraged through particular land use and density distributions, and that many rapidly growing cities have land use and density distributions that are unsupportive. It is estimated that when densification occurs on the periphery of a low-density city, the resulting changes in public transport trip lengths, departure profile and seat renovation can in fact be detrimental to the overall viability of services. The paper demonstrates that population density distribution, land use mix and corridor length can each have considerably stronger causal links to public transport viability, than city population density in isolation. Integrated land use-transport strategies should therefore give equal attention to these three land use characteristics and city population density.