New research from McGill University and the University of California, Santa Cruz has found that the local streets of the world’s cities are becoming less connected, a global trend that is driving urban sprawl and discouraging the use of public transportation.

The new study, published in an open-access paper in Proceedings of the National Academy of Sciences, is the first global history of sprawl as measured by local connectivity of street networks. The research relied on publicly available data sourced from OpenStreetMap, the Wikipedia of maps, and satellite-derived data.

The result of a 7-year collaboration, the study was able to show that in large parts of the world, recent urban growth has increasingly resulted in inflexible and disconnected street networks. Different forms of gated communities were also found to be on the rise globally.

SNDi for streets added years 2000 to 2014. Inset charts the distribution at the country level of the street-network disconnectedness index (SNDi) for streets added in each of the 4 time periods. Barrington-Leigh and Millard-Ball

The pattern of new urban and residential roads represents an essentially permanent backbone that shapes new urban form and land use in the world’s cities. Thus, today’s choices on the connectivity of streets may restrict future resilience and lock in pathways of energy use and CO₂ emissions for a century or more.

In contrast to the corrective trend observed in the United States, where streets have become more connected since the late 20^th century, we find that most of the world is building ever-more disconnected “street-network sprawl.” A rapid policy response, including regulation and pricing tools, is needed to avoid further costly lock-in during this current, final phase of the urbanization process.

—Barrington-Leigh and Millard-Ball

Christopher Barrington-Leigh and collaborator Adam Millard-Ball, an associate professor in the Environmental Studies Department at the University of California Santa Cruz, built a Street-Network Disconnectedness Index to create a global street connectivity map.

Their data showed that southeast Asia is now home to some of the most sprawling cities on the planet – —and sprawl is getting worse. Gridded street networks, on the other hand, promote efficient, dense urban form in Bolivia, Argentina and Peru. Germany, Denmark and the UK have been able to maintain moderate levels of street connectivity due to pedestrian and bicycle pathways, offering greater connectivity to non-motorized travel.

In conjunction with the study’s publication, the authors are launching an online interactive map through which visitors can explore street connectivity around the world. The web site can also be used for making animations showing how street connectivity has changed in a given area.

Past research has shown that the increased accessibility offered by gridded street networks makes walking, cycling and the use of public transit much simpler while cul-de-sacs tend to encourage the use of personal motorized vehicles. Barrington-Leigh said urban planners thus need to think about connectivity at the most local scale when designing and planning new streets in order to make cities more sustainable.

Streets and roads represent an essentially permanent backbone that shapes all other dimensions of urban form and land use. Policy makers should look to cities like Tokyo and Buenos Aires for inspiration on how to limit sprawl. On our current path, choices that limit street connectivity may restrict future resilience and lock in pathways of energy use, CO₂ emissions, health outcomes, and other aspects of our lifestyle for a century or more.

—Christopher Barrington-Leigh

The methodology used to build the connectivity index and maps is described in a paper previously published in PLOS.

This study received financial support from the Social Science and Humanities Research Council of Canada, the Hellman Fellows Program and a UC Santa Cruz Faculty Research Grant.

Resources

• Christopher Barrington-Leigh and Adam Millard-Ball (2020) “Global trends towards urban street-network sprawl” PNAS doi: 10.1073/pnas.1905232116

• Christopher Barrington-Leigh and Adam Millard-Ball (2019) “A global assessment of street-network sprawl”PLOS ONE doi: 10.1371/journal.pone.0223078