By 2050, nearly two-thirds of the world's 9.7 billion people will be living in urban cities, and urban planning is becoming a top priority for builders and architects. Michael Totty, news editor at The Journal Report in San Francisco, highlights five cities that are leading the way in urban innovation as cities expand and the population migrates to urban hubs. The Wall Street Journal asked more than a dozen experts—urbanists, architects, planners—what cities they think are worth watching now. Here is what they discovered:

Singapore: Singapore is “a city innovating under constraint,” says Edward Glaeser, a professor of economics at Harvard University. “You have a limited amount of land—you have to make sure you’re not wasting it.” What deserves the most attention is how it manages its severely limited resources, a situation that cities around the globe will increasingly face.

Houston: From 2010 to 2014, the Texas city added more than 140,000 people, a 6.7% increase and second only to New York in the U.S. But the difference between Houston and other high-growth cities is that it has expanded its housing stock to accommodate its new residents. In roughly the same period, the Houston metro area issued construction permits for 189,634 new units, the most in the nation. It is not surprising, then, that more than 60% of homes in the larger Houston metro area are considered affordable for median-income families, according to the National Home Builders Association, compared with about 15% in the Los Angeles area.

Medellin, Columbia: Like many struggling cities, Medellín has looked to eye-catching building projects to revive its fortunes. What makes this Andean city different, however, is that it has placed some of its highest-profile projects in some of its poorest and previously crime-ridden neighborhoods.

Detroit: The city is proposing a handful of “pink zones,” where red tape will be cut to help small developers and entrepreneurs open new businesses and revive aging commercial strips. The goal is not to eliminate zoning but to ease some of the constraints faced by new projects, like minimum-parking requirements or environmental-impact reports.

Vancouver, British Columbia: The Canadian city may not be the most walkable city in the world, though it regularly makes the top 10. What makes it noteworthy are all the ways it is working to make the city easier, safer and more enjoyable to get around by foot. Roadways are being built and redesigned to favor pedestrians, for example, by installing pedestrian-controlled traffic signals on busy streets.

Seoul and Amsterdam, meanwhile, are among the leaders in putting “smart city” tools into the hands of their citizens.

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