There are neighborhoods where leaving the car outside the house seems like a civilized, almost elegant choice. And then there are areas where that same phrase sounds like a mockery, because without a car it becomes difficult to get to work, take a child to school, reach a clinic, do a decent shopping without turning the day into a small expedition.
Urban mobility is often described in simple words: fewer cars, more buses, more metro, more bikes, more public space. Everything right. Except that you really understand a city when you measure the distance between desire and concrete possibility. The Car Dependency Index, or CDI, tries to do just that: it measures the accessibility gap between private cars and public transport, using high-resolution geospatial data on 18 European and North American cities. The study defines car dependence as a consequence of the urban form, the accessible services and the efficiency of the available alternatives, rather than as a simple individual habit.
The car as a necessity
The CDI works on a very practical question: how many opportunities a person can reach by public transport compared to how many they can reach by driving. “Opportunities” include essential services and daily activities, therefore work, healthcare, education, commerce and places of ordinary life. When public transport allows you to reach fewer destinations, or takes much longer, the index rises. When metro, city trains and trams reduce the gap, the index drops.
The difference is decisive. The Car Dependency Index measures how much a city forces people to use the car, even when they might want to do without it. A person may want a lighter life, less petrol, less traffic, fewer idling around looking for parking. Then he leaves the house and discovers that the first bus passes every now and then, the station is far away, the connection is missed, the journey takes twice as long. At that point the car stops being a vice and becomes a crutch.
The model uses a very fine grid, with hexagonal cells of approximately 200 meters, and compares the opportunities reachable by car with those reachable by public transport. Positive values indicate areas where the car provides more access; the negative ones indicate areas where public transport is more advantageous. The maps show a recurring fact: urban centers tend to have lower dependency, while the suburbs remain more exposed. The presence of urban railway lines, metro and trams can change the situation even outside the centre.
The center runs, the periphery waits
Among the cities analysed, Paris and Zurich are the only cases in which, in the areas considered, public transport offers more opportunities overall than the car. However, the study invites us to read that data with caution, because smaller administrative boundaries can affect the comparison. Milan ranks very well, with an average CDI of 0.063. Rome, however, reaches 0.335, the highest value among the cities considered in the study table.
The Italian comparison weighs heavily. Rome shows a much stronger dependence on the car than Milan, about five times higher if we look at the average values reported in the research. The difference tells of two cities built and served differently. Milan has a denser metro network compared to the urban scale, a more legible railway and tram system, a density that makes it easier to serve many people with frequent lines. Rome occupies an enormous territory, has dispersed peripheral neighborhoods, a less continuous density and a metropolitan network that is still short compared to the size of the capital.
This distinction shifts the discussion from traffic to structure. Rome’s problem also concerns queues, of course, but the CDI photographs something deeper: the city itself, due to the way it has expanded, often makes the car competitive or necessary. In the study’s maps, the Roman metro network appears almost like a visible trace within a much larger territory. Where urban rail arrives, dependency drops. Where the connection is missing, the car wins again for time, comfort and access.
The study also includes a simulation on the expected developments of the Roman metro, in particular new stops on line C and a possible line D. The estimate indicates a reduction of approximately 60 thousand vehicles used for home-work travel, with important benefits in the areas close to the new stations. The result, however, remains localized: to impact the entire city, coordinated interventions on a network scale are needed, capable of stitching together distant areas and neighborhoods that are currently too isolated from strong public transport.
Maps are not enough
The Car Dependency Index also helps avoid a very convenient shortcut: blaming citizens. Those who use cars every day are often described as lazy, backward, allergic to change. In many urban areas, however, the choice is already conditioned. If a mother spends twenty minutes in the car and an hour and a half by three different means of transport, the sermon on sustainability comes late and badly. Before behaviors there are infrastructures, times, distribution of services, the quality of coincidences.
This is where the political utility of the CDI comes from. Such a map can indicate where a limited traffic zone makes immediate sense, because robust alternatives exist, and where it risks shifting the burden onto residents. It can show where interchange parking is needed, where to strengthen a line, where a new stop would have real effects and where an isolated intervention would remain a plaster on a fracture.
To change the way we move we need long, continuous, unspectacular investments. We need tracks, frequencies, protected lanes, exchange nodes, less dispersed neighborhoods, better distributed services. A cultural change is also needed, of course, but culture walks on the legs of infrastructure. Where public transport works, many people take it. Where he arrives every half hour, you greet him from afar and turn the key in the ignition.
A city can ask citizens to use their cars less only when it offers them something instead of keys. A train, a subway, a bus that actually passes. Otherwise it remains a sermon given to those who have yet to reach home.
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