Street-network Sprawl in Washington, United States

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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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Washington: region in United States

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 Washington, United States on the sprawlmap

How have Washington's street networks changed over time?

Most recent snapshot: Taking into account the entire (i.e. aggregate) street network in Washington as of 2014, the overall level of street-network sprawl is 5.05, which is highly disconnected.

Trends in street network construction: The SNDis of street construction for the respective time periods are 3.66, 6.6, 7.37 and 7.19. Street construction in Washington increased in disconnectivity initially but has since improved. The streets constructed in 1991-2000 were the most disconnected.

Quantity of street network construction: The street network in Washington spans a total of 92608 kilometers. It is dominated by roads constructed prior to 1975. These roads have an SNDi of 3.66, which is highly disconnected.

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 3.66, 4.59, 4.86 and 5.05. Overall, the SNDi of the aggregate street network has risen: the street network in Washington has become more disconnected. This increase has slowed: between 1975 and 1976-1990, SNDi rose by 0.92 points, but between 1991-2000 and 2001-2014, it rose by just 0.19.

Washington in context: Street-network sprawl of new development

The level of street-network sprawl in new development in Washington peaked in 1991-2000. To get a sense of how street development has changed in Washington, we can consider three of its most populous cities: Seattle, Spokane and Kennewick. Out of the three cities, Spokane does not follow the same trend as the region. The level of street-network sprawl in new development in Spokane rose steadily.

How do development practices in Washington fare in comparison to others in United States? Most recently in 2001-2014, street construction in Washington was the 4th-most disconnected out of the 46 regions in United States. Its position in the ranks since 1975 has risen; relative to other regions in United States, street construction in Washington has become more disconnected. Washington ranked 8th in 1975, 2nd in 1976-1990, 5th in 1991-2000 and 4th in 2001-2014.

Washington in context: Aggregate street-network sprawl

As of 2014, the city with the most disconnected street network in Washington is Olympia with an SNDi of 4.61, which is highly disconnected. Conversely, the most connected city is Spokane with an SNDi of 1.41, which is relatively well-connected. See trends for these cities: Spokane, Olympia

The level of street-network sprawl in the aggregate network in Washington rose steadily. To get a sense of how the aggregate street network has changed in Washington, we can consider three of its most populous cities: Seattle, Spokane and Kennewick. Disconnectivity in the most populous cities in Washington follow the same region trend.

To date, Washington is the 7th-most disconnected out of the 46 regions in United States. Its position in the ranks since 1975 has risen; relative to other regions in United States, the street network in Washington has become more disconnected. Washington ranked 8th in 1975, 6th in 1976-1990, 6th in 1991-2000 and 7th in 2001-2014.

Cities in Washington

• Olympia
• Seattle
• Yakima
• Kennewick
• Bellingham
• Spokane