🧭 Concept Urban form & morphology Networks & routing Urban Morphology · step 5 Notebook By · Updated

Urban Blocks Explained

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Problem statement

The block — the patch of land enclosed by streets — is the unit that decides how a city works on foot. Small blocks mean frequent junctions, short detours and many routes; large blocks mean long walls, few crossings and trips that go around rather than through. Block size is one of the oldest and most robust measures in urban design, and one of the easiest to compute wrongly, because "a block" depends on which streets bound it and how cleanly the network is drawn.

Quick answer

A block is a face of the street network: a polygon bounded on all sides by street centrelines. Its key measures are:

  • area — the grain of the city;
  • perimeter — the frontage and walking distance around it;
  • blocks (or intersections) per km² — the density of route choices.
import numpy as np, pandas as pd, geopandas as gpd, osmnx as ox, momepy
ox.settings.overpass_url = "https://maps.mail.ru/osm/tools/overpass/api"   # overpass-api.de mirror
ox.settings.overpass_rate_limit = False
from shapely.geometry import Point

ox.settings.http_user_agent = "spatialworkflow-examples/1.0"
PLACES = {"Barcelona Eixample": (41.3928, 2.1633), "Chicago Loop": (41.8837, -87.6325),
          "Den Haag centre": (52.0799, 4.3113), "Rome centro": (41.8986, 12.4768)}

rows = []
for name, (lat, lon) in PLACES.items():
    G = ox.graph_from_point((lat, lon), dist=1000, network_type="drive")
    streets = ox.graph_to_gdfs(ox.convert.to_undirected(G), nodes=False)
    crs = streets.estimate_utm_crs()
    streets = streets.to_crs(crs)
    limit = gpd.GeoSeries([Point(lon, lat)], crs=4326).to_crs(crs).buffer(800)
    blocks = momepy.enclosures(streets, limit=limit)
    blocks = blocks[(blocks.area > 500) & (momepy.elongation(blocks) > 0.15)]
    inner = blocks[blocks.within(limit.iloc[0].buffer(-1))]            # drop edge fragments
    rows.append({"place": name, "blocks": len(inner),
                 "median m²": inner.area.median(), "median perimeter m": inner.length.median()})
print(pd.DataFrame(rows).set_index("place").round(0).to_string())
                    blocks  median m²  median perimeter m
place                                                    
Barcelona Eixample     105    17316.0               518.0
Chicago Loop            93    16988.0               524.0
Den Haag centre         73     8933.0               440.0
Rome centro             67     4196.0               308.0

Barcelona's Eixample and Chicago's Loop come out almost identical, at about 17,000 m². The Eixample figure is larger than its famous 113 m blocks because a block measured between centrelines includes half of each street: the grid repeats every 133 m, and 133² is 17,700 m². Den Haag's centre has half that grain and Rome's a quarter of it. The median block area captures the difference in one number.

Bounding streets, polygonise within a limit, remove non-blocks

1. Choose the bounding streets

Drivable streets define traffic blocks; including footpaths and alleys defines pedestrian blocks, which are smaller. The choice should follow the question.

2. Polygonise within a limit

Blocks come from polygonising the network; a limit polygon keeps the edge of the study area from being lost. Blocks cut by the limit are fragments and should be excluded from statistics.

3. Remove non-blocks

Thin polygons between dual carriageways, traffic islands and roundabout centres are street space. Filter by area and elongation, or clean the network first.

4. Describe the distribution

Median block area and its spread, not the mean: a few park-sized blocks skew means badly.

5. Relate blocks to junctions

Blocks per km² and intersections per km² are two views of the same grain. Report one of them with block size.

A fine grid of small blocks against a coarse grid of large blocks and the walking detour each implies. small blocks, short detoursfine grainmany turns, near the straight linecoarse grainfew turns, long way roundBlock size sets how often a walker can turn towards the destination spatialworkflow.io
Small blocks, short detours; large blocks, long ones.

Detours and traffic against pedestrian blocks

Example 1 — the detour a block imposes

for r in rows:
    side = np.sqrt(r["median m²"])
    print(f"{r['place']:<20} typical side {side:4.0f} m → a walk to the opposite corner "
          f"of the block is {2 * side:4.0f} m along the streets, {side * np.sqrt(2):4.0f} m straight")
Barcelona Eixample   typical side  132 m → a walk to the opposite corner of the block is  263 m along the streets,  186 m straight
Chicago Loop         typical side  130 m → a walk to the opposite corner of the block is  261 m along the streets,  184 m straight
Den Haag centre      typical side   95 m → a walk to the opposite corner of the block is  189 m along the streets,  134 m straight
Rome centro          typical side   65 m → a walk to the opposite corner of the block is  130 m along the streets,   92 m straight

The ratio of route to straight line is the same for every square block; what changes with block size is the absolute detour — and the number of chances to turn.

Example 2 — traffic blocks against pedestrian blocks

lat, lon = PLACES["Den Haag centre"]
for net in ("drive", "all"):
    G = ox.graph_from_point((lat, lon), dist=1000, network_type=net)
    s = ox.graph_to_gdfs(ox.convert.to_undirected(G), nodes=False).to_crs(28992)
    lim = gpd.GeoSeries([Point(lon, lat)], crs=4326).to_crs(28992).buffer(800)
    bl = momepy.enclosures(s, limit=lim)
    bl = bl[(bl.area > 500) & (momepy.elongation(bl) > 0.15) & bl.within(lim.iloc[0].buffer(-1))]
    print(f"{net:<6} network: {len(bl):>4} blocks, median {bl.area.median():>7,.0f} m²")
drive  network:   73 blocks, median   8,937 m²
all    network:  242 blocks, median   3,723 m²

Footpaths through courtyards and parks break traffic blocks into much smaller pedestrian ones. The same city has a coarse traffic grain and a fine walking grain.

Example 3 — the shape of the distribution

lat, lon = PLACES["Rome centro"]
G = ox.graph_from_point((lat, lon), dist=1000, network_type="drive")
s = ox.graph_to_gdfs(ox.convert.to_undirected(G), nodes=False)
crs = s.estimate_utm_crs()
bl = momepy.enclosures(s.to_crs(crs), limit=gpd.GeoSeries([Point(lon, lat)], crs=4326).to_crs(crs).buffer(800))
bl = bl[bl.area > 500]
q = bl.area.quantile([0.1, 0.5, 0.9])
print(f"Rome: 10th percentile {q[0.1]:,.0f} m², median {q[0.5]:,.0f} m², "
      f"90th {q[0.9]:,.0f} m²; mean {bl.area.mean():,.0f} m²")
Rome: 10th percentile 820 m², median 4,941 m², 90th 27,205 m²; mean 14,227 m²

Block areas are right-skewed everywhere: a few very large blocks — parks, palaces, rail land — pull the mean far above the median.

Median block area on the drivable network within 800 m of four city centres. median block area, m², drive networkBarcelona Eixample (105)17,316 — a 133 m gridChicago Loop (93)16,988Den Haag centre (73)8,933Rome centro (67)4,1960900018kCentreline blocks include half of each street around them spatialworkflow.io
Median block area, drivable network, 800 m around each centre.

Block size and walking

Why block size matters for walking

Every block edge is a stretch without a turn. Small blocks give pedestrians many route choices and keep detours short; large blocks force long walks around. Block size and intersection density are among the most consistent built-environment correlates of walking in the research literature.

Why the network definition dominates the answer

Blocks are only as good as the lines that bound them. Omitting footpaths merges pedestrian blocks; mapping dual carriageways separately creates slivers; unconnected lines leave blocks open. Two studies of the same city can report very different block sizes for these reasons alone.

Why the median, not the mean

A few non-residential blocks — a park, a hospital campus, a railway yard — can be twenty times the typical block. The median describes the typical grain; the mean mostly describes the outliers.

Typical block side and the walk to the opposite corner of a block in four city centres. what the grain means on footcentreblock sidearound the blockstraight lineBarcelona Eixample132 m263 m186 mChicago Loop130 m261 m184 mDen Haag centre95 m189 m134 mRome centro65 m130 m92 mThe ratio is the same for any square block; the detour and the number of turns are not spatialworkflow.io
Rome’s blocks put a corner every 65 m; the Eixample and the Loop every 130 m.

Edge blocks, superblocks and courtyards

  • Edge blocks clipped by the study limit are partial; exclude them.
  • Superblocks (Barcelona's traffic-calmed groups of nine blocks) are a design unit above the block; the street network still shows the original grid.
  • Courtyard blocks contain private open space that is not visible in the street network.
  • Water and rail should bound blocks too; pass them as additional barriers.
  • Units: report area in m² or hectares consistently.

FAQ

What is an urban block?

The area enclosed by streets on all sides — a face of the street network.

How big is a typical city block?

It varies from under 5,000 m² in old European centres to well over 10,000 m² in many American grids.

How do I compute blocks from OSM?

Polygonise the undirected street network within a limit, for example with momepy.enclosures.

Why use the median block size?

Block areas are heavily right-skewed; the median describes the typical block.

Do footpaths count?

For pedestrian blocks, yes. Report which network defines the blocks.

Smaller blocks mean more junctions, shorter detours and more route choices.