In the next 12 years, 1.5 million newcomers are expected to move to the San Francisco Bay Area. Sprawl threatens to gobble up a tenth of the region's remaining green spaces. Steinbach, 42, and his team at Greenbelt Alliance (greenbelt.org), a nearly 50-year-old Bay Area–based conservation group, are armed and ready. He recently helped persuade San Jose to keep the South Bay's Coyote Valley from becoming an industrial campus. His tactic: pressing outlying suburbs to direct their development inward, focusing their growth around existing downtowns and public-transportation hubs. "We see protection of open space and the creation of more livable, walkable cities as two sides of the same coin," says Steinbach. "The more you do of one, the more you get of the other."

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