For example, green building programs encourage construction of energy-efficient housing in areas with easy access to jobs, services, shops, and public transportation. From a climate change perspective, the point is to reduce driving and therefore cut greenhouse gas emissions.

Extend the philosophy to affordable housing programs, and the benefits multiply. Residents get better access to training and job programs and more options about how to spend their limited incomes. The better transportation options are a particular benefit: A study of 28 metropolitan areas found that families with incomes between $20,000 and $50,000 spend an average of 29 percent of their income on transportation, compared to the national average of 19 percent.