Recent data from Caltrans shows the varying effects of building housing within 1/4 mile of frequent public transportation. Basically, the lower the income of the household, the higher the impact of living close to transit. Affordable apartments in, say, downtown Menlo Park would achieve tremendous greenhouse gas benefits. Placing them within walking/biking distance of work would boost the payoff even more. See the recent analysis by the Center for Neighborhood Technology.
In short, affordable housing benefits everyone. Based on conservative assumptions, investing just 10% of cap-and-trade revenue in California's Transit-Oriented Development Housing Program for the next three years will result in 15,000 affordable homes. That translates into 105,000,000 fewer miles of vehicle travel per year on our roads. Statewide, that's 1.58 million metric tons less of greenhouse gas pollution...and cleaner air for all.