Street-network Sprawl in Seoul, South Korea

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What is Street-network sprawl?

Street-network Sprawl is a way to measure urban sprawl, worldwide, through the connectedness of the streets. Less sprawl means more connected, more walkable streets. Well-connected streets – like New York City’s grid – are more walkable and can be served by public transit.

The street network is permanent, and its connectivity affects the livability and environmental footprint of cities for decades and centuries to come. In places with more connected streets, residents drive less and walk more. A well-connected street network is associated with better outcomes for health, the environment, sustainable consumption, social integration, and equity.

We can quantify how connected street networks are with the Street Network Disconnectedness Index (SNDi).

SNDi -- our measure of street-network sprawl (disconnectedness)

The SNDi is a comprehensive measurement of “sprawl”. It captures:

• nodal degree
• dendricity, the tree-likeness of the street network
• circuity, the ratio of the distance by road to the straight-line distance of two locations on the street network
• sinuosity, the curviness of a street

A higher SNDi means less-connected streets – i.e., more sprawl. For the 10137 cities in our dataset, the average SNDi is 2.25, with half of the cities' SNDis falling between 1.08 and 3.25.

More information on the sprawl index can be found in these research papers:

• Global trends toward urban street-network sprawl
• A global assessment of street-network sprawl

To see the state of street-network sprawl across the globe, visit the sprawlmap.

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Seoul: city in South Korea

What exactly constitutes the spatial extent of the city? For these aggregations, we used the Global Human Settlement Layer Urban Center Database (GHS-UCDB) to define the boundaries of the city. These cities -- or urban centers -- cover areas that are densely populated and built-up, and so may extend beyond the spatial borders of these cities that we may be familiar with. The GHS area is shaded in blue.

View Seoul, South Korea on the sprawlmap

How have Seoul's street networks changed over time?

Most recent snapshot: Taking into account the entire (i.e. aggregate) street network in Seoul as of 2014, the overall level of street-network sprawl is 1.13, which is relatively well-connected.

Trends in street network construction: The SNDis of street construction for the respective time periods are 0.89, 1.59, 2.29 and 2.82. In each period, new street development in Seoul steadily grows more disconnected than in the last. This increase has slowed: between 1975 and 1976-1990, SNDi rose by 0.69 points, but between 1991-2000 and 2001-2014, it rose by just 0.53.

Quantity of street network construction: The street network in Seoul spans a total of 22463 kilometers. It is dominated by roads constructed prior to 1975. These roads have an SNDi of 0.89, which is relatively well-connected.

Effect on the aggregate network: New construction in each period adds to the total stock of streets, but does not change streets that have already been built. Therefore, it has a limited effect on the street network as a whole. The SNDis of the aggregate street network in the respective time periods are 0.89, 1.02, 1.08 and 1.13. Overall, the SNDi of the aggregate street network has risen: the street network in Seoul has become more disconnected. This increase has slowed: between 1975 and 1976-1990, SNDi rose by 0.13 points, but between 1991-2000 and 2001-2014, it rose by just 0.05.

Seoul in context: Street-network sprawl of new development

Seoul and Seoul do not follow the same trend in the disconnectivity of their street network constructions. The SNDi in Seoul rose steadily, while the SNDi of street constructions in Seoul peaked in 1991-2000.

How do development practices in Seoul fare in comparison to others in Seoul? Most recently in 2001-2014, street construction in Seoul was the 1st-most disconnected out of the 1 cities in Seoul. Its position in the ranks since 1975 has not changed. Seoul ranked 1st in 1975, 1st in 1976-1990, 1st in 1991-2000 and 1st in 2001-2014.

Seoul and South Korea follow the same trend in the disconnectivity of their street network constructions. The SNDi for both of these rose steadily.

How do development practices in Seoul fare in comparison to others in South Korea? Most recently in 2001-2014, street construction in Seoul was the 14th-most disconnected out of the 39 cities in South Korea. Its position in the ranks since 1975 has fallen; relative to other cities in South Korea, street construction in Seoul has become more connected. Seoul ranked 11th in 1975, 14th in 1976-1990, 11th in 1991-2000 and 14th in 2001-2014.

Seoul in context: Aggregate street-network sprawl

Seoul and Seoul follow the same trend in the disconnectivity of their aggregate street networks. The SNDi for both of these rose steadily.

To date, Seoul is the 1st-most disconnected out of the 1 cities in Seoul. Its position in the ranks since 1975 has not changed. Seoul ranked 1st in 1975, 1st in 1976-1990, 1st in 1991-2000 and 1st in 2001-2014.

Seoul and South Korea follow the same trend in the disconnectivity of their aggregate street networks. The SNDi for both of these rose steadily.

To date, Seoul is the 16th-most disconnected out of the 39 cities in South Korea. Its position in the ranks since 1975 has fallen; relative to other cities in South Korea, the street network in Seoul has become more connected. Seoul ranked 11th in 1975, 15th in 1976-1990, 16th in 1991-2000 and 16th in 2001-2014.

What about other similarly-populated cities?

As of 2015, Seoul had a built-up area of 1079.29 square kilometers, and a population of 21600841 people.

These are some other cities with approximately the same population:

• Kolkata, India
• Cairo, Egypt
• Mexico City, Mexico

For some related information about population, urban extent and density, and roads, visit the Atlas of Urban Expansion.