How to reduce U.S. greenhouse-gas emissions? Building compact, mixed-use neighborhoods would be just as effective as much-touted policies like boosting fuel economy, cleaning up power plants, and building green, says a new analysis from the Urban Land Institute. The U.S. population is expected to grow 23 percent by 2030; under the sprawl-encouraging status quo, driving is expected to increase 59 percent in the same time period. But it doesn’t have to be that way, says the ULI: some two-thirds of homes and other buildings expected to be needed by 2050 have yet to be built, and they don’t have to be part of outward-oozing communities. According to the report, walkable neighborhoods can decrease driving by up to 40 percent — which would reduce CO2 emissions, help save our arses, and perhaps reduce our arses as well.