Extended ecological footprint for different modes of urban public transport: The case of Vienna, Austria

Urbanization and population growth in urban areas are linked to increasing passenger transport and decreasing land availability. One option to cope with the negative impacts associated to this growth (i.e. emissions from and land use by traffic) is to strengthen public transport, as it has lower land requirements and higher transportation capacities if compared to private passenger transport by cars. Besides the direct land use within the city borders, transportation systems also cause land use in the hinterland, particularly for the extraction of raw materials, for energy supply, and for the sequestration of greenhouse gas emissions. The study at hand investigated these types of land uses of a multimodal public passenger transport network consisting of subway, tram, and bus transport, taking the case study of Vienna. The land uses distinguished were the direct land use in the city, the direct land use in the global hinterland to provide energy and resources, and the land needed to sequestrate the CO2 emissions emitted. For the latter a distinction between the CO2 emissions from energy consumption (operational energy CO2 hinterland use), and from CO2 embodied in goods and materials (embodied CO2 hinterland use) was made. The overall land use of the public transport system was finally determined and illustrated using an extended ecological footprint (EF) analysis under consideration of the life cycle of used goods and materials. Results were expressed in global hectare (gha/a) for one year and further normalized to the transport capacity and performance of each transport mode. Results indicate that the operational energy CO2 hinterland use contributes most to the overall land use (55,000 gha/a), followed by the embodied CO2 energy hinterland use (15,000 gha/a), the direct hinterland use (1,660 gha/a) and the direct land use within the city (620 ha). This sums up to a total of 72,500 gha/a, which, considering Vienna's population of 1.8 million inhabitants, equals 0.03 gha/capita.a. The direct land use within the city corresponds to 1.5% of city area and 1% of the EF. Divided by transport mode, the subway has the largest EF (51%) followed by busses (20%), trams (19%), and services (10%). However, the ranking changes when the transport performance is considered. In general it can be taken from the results that the specific environmental efficiency (specific land use per seat kilometer provided) is increasing with growing offer of service per route. Due to the fact that infrastructural and non-operational energy impacts (e.g. construction materials, station lighting and heating) are not increasing substantially with a higher succession of trains the effect is even higher by rail-bound systems. However, if the required transport capacity per hour falls below a certain limit, subways and trams are not only economical, but also environmental less efficient than bus systems.