Just as you might expect, highways cause air pollution.

Or, to be more precise: all else being equal, places with more highway miles per capita tend to produce higher levels of air pollution from vehicles, because people drive more.

Among the 66 large cities ranked in terms of miles of highway lane per capita, Seattle and Portland were 44th and 45th, respectively. Not bad, but the highway miles per capita in the Northwest were double the amount in New York City (that Ecotopia on the Hudson).

And among 106 smaller U.S. cities, Eugene ranked 32nd, Spokane 41st, and Boise 66th in highway lane miles per capita.

The good news is that, mile for mile, today’s vehicles produce from 80 to 99 percent less pollution than did vehicles in the 1960s. The bad news is that there are lots more vehicles, and we drive them much more. In many ways, our air is cleaner than it used to be—but it could be far, far cleaner still.