The trade-off between greenhouse gas emissions and accessibility for public transport: the case of Turin

Local public transport can contribute to the decarbonisation of urban mobility, by providing an alternative to private cars for commuting and other daily mobility needs. The modal choice of local public transport is intrinsically related to its accessibility and capillarity on the territory, so that the expansion of existing routes can be regarded as an effective policy to realise significant emission savings. However, enhancing the accessibility (hence, the number of runs of public transport means) would lead to increased fuel consumption and, consequently, a rise of local and global emissions, so that the trade-off between the provision of an adequate level of service to all citizens and the emission savings attainable from the switch from private mobility to public transport should be carefully evaluated. This analysis builds on monitored data for the city of Turin, in Northern Italy, on the operation of urban buses and their fuel and electricity consumption, with the aim of analyzing the levels of service across neighborhoods and the related emissions. The performance of the service is evaluated on the basis of the well-established Public Transport Accessibility Level (PTAL) by means of GTFS data, a standard that allows for the geographical analysis of bus routes, comparing scheduled trips and actual trips. In parallel, the fuel consumption is estimated by considering real monitored data for the buses operating in Turin over one year, calculating average fuel consumption indicators, and compared with nominal consumption figures provided by manufacturers. The analysis compares alternative technologies, including diesel, compressed natural gas and battery electric buses, also incorporating the contribution of renewable energy such as biodiesel, biomethane and renewable electricity generation. The emissions resulting from the operation of public transport means are then compared to the counterfactual scenario in which the same level of service is provided by private motorized mobility, under different assumptions of bus load factors. The aim of the study is to discuss the trade-off between the accessibility to public transport and the generated environmental impact, by providing quantitative indicators to support policy actions and other strategies at the urban level.