## 1 Introduction
### The Environmental Impacts of Urban Transport  
Transport is one of the economic sectors exerting the greatest pressure on environmental and natural resources. The decoupling of transport growth from economic growth and modal rebalancing are among the qualifying objectives for sustainable mobility. The environmental indicators used to assess the status and trend of the transport sector include energy consumption, greenhouse gas emissions (carbon dioxide, methane and nitrous oxide), atmospheric pollutant emissions (nitrogen oxides, non-methane volatile organic compounds, particulate matter, lead, benzene and sulphur oxides), and the share of the vehicle fleet complying with certain emission standards. Another key indicator is the demand and intensity of passenger transport, which measures the demand for passenger transport (broken down by the different modes of transport) and its intensity also in relation to economic growth trends. In Europe, the transport sector is responsible for almost a third of final energy consumption and about a quarter of total greenhouse gas emissions [15]. In 2018, the energy used for transport (considering road, rail, maritime, and aviation) was 393,143 ktoe, corresponding to 28% of final consumption (authors' calculation from [16]). Road transport, in particular, is responsible for the largest share of energy consumption in the transport sector. In the 27 EU Member States (EU-27), it accounted for about 93% of total demand in 2019, excluding international aviation and maritime transport, an increase of about 34% since 1990 (authors' calculation from Eurostat data, available in [17]). Regarding greenhouse gases (GHG) emissions, transport is, together with energy industries, the most responsible emitting sector. From 1990 to 2019 in the EU-28 countries, GHG emissions from the transport sector increased by 20.3%, although they decreased in the period 2008-2013. Comparable figures apply to Italy. The transport sector was responsible for 25.2% of total national GHG emissions in 2019 [15], of which, excluding international aviation and international shipping, 92.6% are from road transport. Specifically, 68.7% of national GHG emissions came from passenger cars, 15.4% from heavy duty vehicles, 10% from light commercial vehicles, 3.3% from buses, 2.6% from motorbikes and 0.3% from mopeds. Road transport in 2019 was also responsible for 40.3% of nitrogen oxides emissions, 11.4% of NMVOC emissions, 10.1% of PM2.5 fine particulate emissions, and 18.7% of carbon monoxide emissions over the total emitted in Italy [15; 18]. However, it is important to remember that although carbon emissions cause the same damage no matter where (and when, to a certain extent) they are emitted, the place and time of local pollutants emissions is crucial. The emission of pollutants in high-density urban environments is thus even more harmful. Over the period 1995-2019, passenger transport demand increased by 17.4% in Italy, with more than 90% road transport in 2019, predominantly met by individual transport (cars and motorbikes) [15; 18]. In 2018, cars accounted for 76.5% of the modal split of passenger-km, compared to 5.7% use of rail and 10.9% of buses [19])+. The motorisation index in Italy is also very high, with 666 cars per 1000 inhabitants in 2020 [20], the second-highest among EU countries after Luxembourg. On the basis of available data [21; 22], we estimate that in 2019, 42.1% and 94.5% of the consumption of diesel and petrol, respectively, is due to passenger cars, which account for 68.9% of road consumption, with approximately 410,000 million vehicle-km, representing 80.8% of the total. After reaching its highest level around 2007, the evolution of passenger transport intensity (which is measured as passengers per kilomet in relation to population) in Italy had sharp reductions in 2011-2012 due to the economic crisis in those years and a subsequent increase between 2013 and 2016. In 2017 it reached the value recorded in 2009, also maintained in 2019 after a slight decrease in 2018.