This interactive visualisation accompanies the research paper Car Dependency in Urban Accessibility by Campanelli, Marzolla, Bruno, Melo & Loreto (2026).
What is the CDI?
The Car Dependency Index measures, for every ~200 m hexagonal cell in a city, how much more reachable opportunities are by private car than by public transport. It is defined as:
CDI = (Ocar − OPT) / (Ocar + OPT)
Where Ocar and OPT are the opportunity scores reachable by car and public transport respectively.
Reading the scale
CDI < 0 (blue) — Public transport reaches more opportunities than the car. A car-free life is realistic.
CDI ≈ 0 (white) — Both modes offer comparable access.
CDI > 0 (red) — Cars dominate. The greater the value, the more dependent residents are on private vehicles.
How to read the visualisation
The map (left panel) shows the city divided into ~200 m hexagonal cells, coloured by their CDI value on the diverging blue–red scale. Use the toggle in the top-right of the map to switch to a Dorling cartogram, where each cell is resized by its resident population — empty hexes shrink to nothing, and densely populated ones balloon. The scatter plot (right panel) places each hexagon by its opportunity score by car (x-axis) and by public transport (y-axis). Hexagons on the diagonal are balanced; those below it are car-dependent.
Data & methods
Cities are covered with H3 resolution-9 hexagons. Walking and driving times are computed via OSRM on OpenStreetMap data; public transport via GTFS feeds and the Connection Scan Algorithm. Driving times include a parking buffer and city-specific traffic delays. POIs from OpenStreetMap. Population from WorldPop.
Reading the scatter plot
Each dot is a hexagonal cell. Its position encodes:
X-axis (Car): POIs reachable within ~60 minutes by private car.
Y-axis (Public transport): POIs reachable within ~60 minutes by transit.
The dashed diagonal marks CDI = 0: equal access by both modes. Points below the line are car-dependent (red); above are public-transport-favoured (blue).
Dot colour shows the CDI value on the same blue–red diverging scale used on the map.
Hover or click on a dot to cross-highlight it on the map.
Reading the map
Each hexagon (~200 m) is coloured by its CDI value:
−1 PT-favoured0 Balanced+1 Car-dependent
Deep blue = public transport reaches far more opportunities than driving. Deep red = the car is essentially required to reach the city's amenities.
Use the slider in the sidebar to focus only on hexagons within a CDI range — useful for spotting either car-trapped or transit-rich areas.
Click any hexagon to inspect its full breakdown. Hover to cross-highlight it on the scatter plot.
Car Dependency IndexA spatial metric that compares opportunities reachable by car and by public transport. Blue cells favour transit; red cells require a car.