Models and scenarios for sustainable mobility in cities

In the pursuit of carbon neutrality, we have the need and the opportunity to rethink the way we live. In particular in cities, where the majority of carbon emissions are generated, the way we move is a crucial aspect that can be reimagined. This work contributes in this effort, using tools and approaches derived from physics and complexity science. We combine data from various sources with modelling to evaluate the effectiveness of a urban planning paradigm in promoting sustainable mobility. We cross data and simulations to assess the feasibility of getting rid of cars from case-study cities without compromising accessibility to opportunities, and we evaluate the spillover effects on the cities' economic fabric of the construction of new public transport infrastructures, through a case study. The urban planning paradigm whose effectiveness we addressed is the proximity city one, embodied among others by the \emph{15-minute city}. In the strive for more environmentally sustainable urban areas, the 15-minute city has been proposed to encourage active mobility, primarily through walking and cycling. An urban area is considered a 15-minute city if every resident can access essential services within a 15-minute walk or bike ride from their home. Despite being conceived and celebrated for promoting sustainability, there is an ongoing debate regarding its effectiveness in reducing car usage and, subsequently, emissions in cities. In this study, we conduct a large-scale data-driven analysis to evaluate the impact of service proximity to homes on CO$_2$ emissions. Comparing 700 cities worldwide, we find that indeed cities with services in proximity to residents' homes emit less CO$_2$ per capita for transport. By examining nearly 400 cities worldwide, we show that also within the same city neighbourhoods with services located closer to residents' homes produce less CO$_2$ emissions per capita from transportation. We find a relationship between the proximity of services and CO$_2$ emissions for each city. Additionally, we quantify the potential reduction in emissions for 30 cities if they would ideally relocate their services to adhere to the 15-minute paradigm. In the vast majority of cities, we predict a decrease in emissions in such scenario. We moreover find a relationship between the average proximity of services to home locations in cities and their car use share. Through an agent-based model we propose an explanation of this trend in terms of a constant energy budget people are willing to allocate for mobility in a usual day. These findings globally highlight the effectiveness of urban planning based on proximity of services from home locations, especially the 15-minute city, in promoting sustainable mobility. We also observe that among cities with similar average proximity of services, cities spreading over larger areas tend to emit more. We argue that this is due to long-range mobility, towards services that cannot be provided in proximity to every citizens' residence. To provide accessibility to these opportunities at the whole city scale, together with ensuring its cohesion, cars and public transport are the primary modes of mobility. We propose an index to evaluate car dependency in urban areas based on the comparison between precisely the accessibility of opportunities by public transport and by car. Using numerical simulations, we map its spatial distribution in 11 case-study cities. We also observe that cities with a higher average car dependency tend to have a larger share of car use. Proposing this metric, we also establish the foundations of a quantitative framework that policy makers can use to evaluate the feasibility of urban planning interventions. In Vienna, as a case-study, we cross our car-dependency metric with socioeconomic indicators, at the intra-city scale. We find that there the car dependency burden does not lie primarily on districts inhabited by lower-income households, because its spatial distribution seems not to feature correlations with districts' average income. We most notably find that in Vienna car ownership is driven also by car dependence: for comparable average incomes, residents in districts that feature higher car dependency tend to own more cars. Finally, we observe that in Vienna, when access to a private car is taken into consideration, the average accessibility to opportunities decreases significantly and the inequality of its distribution among citizens significantly increases. To provide long-range accessibility in cities in a sustainable way therefore we should switch from the use of cars to public transport. The last chapter of the thesis explores the spillover effects on economic activities of the strengthening of public transport systems, through the analysis of a case study: the construction of a new metro line in Rome. We measure the increase in accessibility brought by the new infrastructure in the neighbourhoods it connected. We also find a significant increase of the number of economic activities in these neighbourhoods. We find a relation between the increase in accessibility and the enhancement of the economy among neighbourhoods, suggesting that economic activities benefited from the connection the new metro provided. %We parallelly observe a decrease in the diversity of kinds of economic activities neighbourhoods host, and therefore of the services they provide, after the construction of the metro, We observe a simultaneous slight decrease in the diversity of economic activities in neighbourhoods and, consequently, in the range of services they provide after the construction of the metro. This can be interpreted as indication of clustering of activities of the same kind. In micro-enterprises, we also highlight an increase in employment opportunities and in the GDP generated as a result of the introduction of the metro line. This study sheds light on the potential benefits also for the city's economic fabric of the implementation of measures necessary for transitioning towards a sustainable mobility. This thesis contributes to the literature on sustainable mobility by evaluating the effectiveness of proximity-based urban planning paradigms in reducing carbon emissions and car use, proposing and discussing the results of an estimation of car dependency in urban areas, and highlighting potential positive spillover effects of urban transport improvements through a case study.