Mass transit has, according to its fans, a staggering array of benefits. It reduces pollution, improves quality of life, and anchors vibrant walkable communities. It boosts public health and makes people happier. But relatively few transit-boosters understand that existing federal guidelines for assessing which new projects to fund not only exclude those considerations, they make it extremely difficult for newly built transit to meet those objectives. A new proposed rule from the Department of Transportation, now entering its 60-day comment period to let people raise objections, should change all that for the better.

The existing rule, sadly, evaluates proposals almost exclusively on the basis of how much time a new rail line shaves off commutes. But taking a train station-to-station rather than driving a car door-to-door is guaranteed to be slower unless traffic jams are severe. This has biased new mass-transit construction in favor of a very particular kind of project: First identify a highway that’s already extremely congested and where widening it is either politically or logistically impossible. Then build commuter-rail tracks in the highway median. Put the stations far apart from one another so that trains can cruise at maximum speed for a long time. Surround the stations with parking lots. Driving your car to a park-and-ride station and taking the train downtown is now cheaper and perhaps faster than the average trip on the congested highway.

That’s how you get something like the Washington, D.C. Metro’s Orange Line in Fairfax County, Va. Trains run in the I-66 median, and stations are far apart and surrounded by open-air parking lots. It’s a helpful addition to the region’s commuter mix. But it doesn’t create the kind of transit-oriented neighborhoods that you see all over Brooklyn or Boston.

For that, you need something else entirely. You need train stations surrounded by densely built structures, not parking lots. And you need the stations to be relatively close to each other. Dense building around one station creates a little circle of walkability. A series of stations built close together, each surrounded by its own little circle, creates a string of walkable pearls that re-enforce each other. That’s how you get whole new communities where people get by with fewer than one car per adult, spurring a circle of demand for pedestrian-oriented businesses and decreased demand for car ownership. And that’s what gives you big public health and environmental benefits.