The mass adoption of gasoline-powered cars over the last century or so probably represents the single greatest threat to public health in the long term (and a fairly large threat to public health in the short term, too), but that's not the only reason that we should stop driving.
So, we can all agree that cars, and car-driven public policy, is bad for us in the sense that it is actively destroying the planet and reinforcing unhealthy sedentary lifestyles. (And, again, that's not even mentioning that motor vehicle accidents are among the leading nonmedical causes of death across all age groups.)
As it turns out (and as many of us could have guessed), cars, and urban planning that reifies and institutionalizes the car as the default mode of urban transportation at the expense of public transportation, are also terrible from the point of view of income inequality, as The New York Times learns from a new study that compares income mobility—the ability of people to move from one income bracket to another—across metropolitan areas:
All else being equal, upward mobility tended to be higher in metropolitan areas where poor families were more dispersed among mixed-income neighborhoods. [...]
The comparison of metropolitan areas allows researchers to consider local factors that previous mobility studies could not — including a region’s geography. And in Atlanta, the most common lament seems to be precisely that concentrated poverty, extensive traffic and a weak public-transit system make it difficult to get to the job opportunities. “When poor communities are segregated,” said Cindia Cameron, an organizer for 9 to 5, a women’s rights group, “everything about life is harder.”
Among other things, the study can and should be read as a powerful warning to cities like New York, where rising income inequality and an out-of-control housing market increase income segregation. But it's important to remember that New York is already so dense that income segregation by neighborhood within the limits of the city—which is served by a robust and nearly comprehensive public transportation system—still allows for a more integrated, and therefore more income-mobile metropolitan area. (That being said: New York won't be safe until we kill the rich and murder the landlords.)