Street Growth Screensaver
Street Growth is a free screensaver that draws a city's real street network outward from its centre like a time-lapse, main roads first and local streets last, for 27 cities, and ends by showing how many kilometres of road were drawn. It uses OpenStreetMap data and ESC exits.
How the Street Growth screensaver works
The site's data pipeline asks the Overpass API for OpenStreetMap highways of seven types — motorway, trunk, primary, secondary, tertiary, residential and unclassified — within about 4 km of a central point in each of 27 cities. Ways are cut at the edge of a circle, simplified so that corners within about 6 m disappear, snapped to a 5 m grid relative to the centre and packed into a compact file.
The reveal is a distance sweep, not a history. The saver sorts road segments by their distance from the centre point and draws them outward in three passes: motorways, trunk and primary roads first, then secondary and tertiary roads, then residential and unclassified streets. A bright front marks the segments just reached.
Every figure on screen is arithmetic on the packed geometry. The kilometre counter adds the lengths of the segments as they are drawn and ends on the city's total, and the class legend on wide screens shows the running kilometres per road class. Dual carriageways are separate ways in OpenStreetMap, so they count twice.
Because the file has a size budget, each city is shown over the largest radius from 4.0 down to 1.5 km that fits it. Dense cities such as London are shown over a smaller circle than sparse ones, so maps are not at the same scale; the radius and a scale bar are printed on screen.
About the data and the licence
The streets come from OpenStreetMap, a map of the world edited by volunteers, read through the public Overpass API (overpass-api.de) once per city. Data © OpenStreetMap contributors, available under the Open Database Licence (ODbL) 1.0; the licence requires attribution, which the saver prints on screen, and the same credit applies to any copy of the file.
The centre of each city is a single well-known point such as a main square or station, chosen by hand, so two people could reasonably pick a different centre and see a different map. The pipeline checks that each city has at least 150 ways and between 3 and 60 km of road per square kilometre, and that the packed file decodes back to exactly the same geometry before it is saved.
The reveal order is an artistic device that makes the structure of a network visible, from arterials to side streets. It carries no information about when any road was built.
A worked example
Within 3 km of its centre point, Paris has 4,531 mapped road segments totalling 404.7 km of centreline, about 14.3 km per square kilometre: 61 km of primary roads, 108 km of secondary and tertiary roads and 236 km of residential and unclassified streets, with no motorway or trunk road inside the circle. As a loose plausibility check, one published figure puts the whole city, 105 km² including its woods, at roughly 1,600 km of streets, about 15 km per km². Mexico City over 3.5 km comes to 792 km, the most of the 27 maps, and Mumbai over 4 km to 347 km, in part because the circle includes sea.
Settings & tips
- City holds one city on screen; the default cycles through all 27.
- Style switches between light-on-dark, amber and blueprint. Amber and blueprint use their own background unless you change the background colour.
- Colour roads by class turns the class colours on or off; off, the classes differ only by line weight and strength.
- Speed sets how long each sweep takes and how long the finished map is held.
- Press ESC to leave fullscreen.
Frequently asked questions
- Is this how the city actually grew?
- No. OpenStreetMap does not give a simple construction date for roads, so the animation draws them in order of distance from the centre, with main roads first. Real cities grew unevenly, often along rivers, rail lines and older roads, and some of their oldest streets are not near the middle at all.
- Where does the map data come from, and can I reuse it?
- From OpenStreetMap contributors through the Overpass API. The data is under the Open Database Licence (ODbL), which requires the credit "© OpenStreetMap contributors"; it is printed on the screen at every size.
- Why are some cities drawn over a smaller circle than others?
- The data file has a size budget, so each city gets the largest radius between 4.0 and 1.5 km that fits. Dense cores such as London and Melbourne end up at 2 km while sparser maps run to 4 km. The radius and a scale bar are shown on screen.
- Which roads are missing?
- Service roads, tracks, footpaths, cycleways, pedestrian streets and one-way link ramps are left out, and corners within about 6 m are smoothed, so the kilometre figure is lower than the length of every mapped way. Dual carriageways are two ways and count twice.
- Is OpenStreetMap complete in every city?
- No. It is volunteer-mapped and coverage varies. The saver checks only that each city has a plausible number of ways and a plausible density; it cannot tell you a street is missing. Low densities, as in Cape Town and Mumbai, can also reflect sea or large open areas inside the circle.
- How current is the map?
- The pipeline re-fetches the maps about every three months, so recent changes in OpenStreetMap can take that long to appear.