Making room through urban expansion matters because people need an adequate supply of developable and accessible land on the urban periphery, sufficient to keep housing with good access to jobs affordable. It also allows municipal authorities to guide development, rather than follow it, thereby greatly reducing the cost of infrastructure and public open spaces while ensuring that new areas on the urban periphery are served and can participate in the city’s economy. 

We believe that the making-room approach—through effective densification and orderly, climate-sensitive urban expansion—is the way to create cities that are more equitable, efficient, livable, and sustainable. 

Past attempts at city densification failed because they focused on containing expansion rather than making room. The Urban Expansion Program recognizes that, unless cities make adequate room for effective densification, they will need to make room through expansion. Through our detailed analysis of the Anatomy of Density—decomposing urban density into its factors—we can design comprehensive programs for making room for densification, which we are testing in pilot cities.

That said, our studies show that, on average, one-quarter of the population added to cities in the last quarter century was accommodated through densification, while three-quarters was accommodating by making room for urban expansion. To make room for urban expansion, we recommend that cities take four key simple steps: 

• First, make realistic 30-year projections of the amount of land that the growing urban population will require, locate that land, and obtain official jurisdiction to that land;
• Second, plan an arterial infrastructure grid—consisting of the rights-of-way for 30-meter-wide arterial roads spaced 1-kilometer apart—and secure the land for that grid, marking it with “stakes-in-the-ground” or with trees along its future sidewalks; 
• Third, identify large public open spaces, including areas of environmental risk, and adopt strategies to protect them from development; and
• Fourth, ensure that land developers—both formal and informal—have the incentives and skills to engage in the proper subdivision of new developments into regular street layouts.

The Urban Expansion Program is a capacity-building program. It provides municipalities with training and tools to prepare and implement their expansion plans by themselves. We have assisted cities in Ethiopia and Colombia with their urban-expansion initiatives; in both countries, a number of cities finalized their plans and secured the rights-of-way for arterial grids on their urban peripheries. Cities in Ethiopia built tens of kilometers of new arterial roads. Following their lead, the national governments in both countries worked with us to expand the initiative to more of their fast-growing cities and created national programs.