One oft-cited California study predicts that within 20 years, 15 million American households will want homes within walking distance of mass transit. If that means living near rail, then many people will be disappointed. With only 2,900 light, heavy, and commuter rail stations currently on-line around the country, simple math tells us that meeting this level of transitoriented development demand would require wrapping dense communities averaging about 5,000 homes around each and every station.

As we all know, that's never going to happen. Getting approval is a problem, of course. But in the years to come, the real bottleneck for transit is in available land. Development sites in transit zones have become the new beachfront property: prized, scarce footage with scant new supply.

In a majestic country of some 2 billion acres, talk of any land supply shortage might seem a little premature. But even allowing for a modest increase in stations, only 300,000 of those two billion acres are within walking range of rail transportation. And with many of those acres already built-up, undevelopable, or devoted to untidy necessities like parking, development land in transit station zones has become highly desirable, very rare, and extremely expensive.

Meanwhile, ridership is growing and land planners are working overtime. A Web search for “transit oriented development” yields 592,000 hits, or about 200 for every actual rail station. Every urban and suburban jurisdiction within eyesight of rail transit is looking for mixed-use transit-oriented opportunities. Industry conferences and investor conversations also stress the importance of investing in transit and transit-oriented development. And real estate investment is a leading indicator of future living and travel patterns.

Looks like the turnstiles are getting crowded at the transit-land market.

But surely the supply of transit-oriented real estate is more elastic than of beachfront property. When supply gets tight and prices go up, won't the market simply respond by creating more supply? That's what an economics textbook might say.

Land supply is tough to create, though, both on the coasts and in transit zones. One way to make new transit-zone land is to build new stations and (often) miles of new track. New rail line extensions and stations cost tens of millions, hundreds of millions, or even billions of dollars. As a result, these important public investments usually take decades of planning and concentrated political will to implement. Many rail corridors were established more than a century ago, and it's getting harder to find new places to put rail over, under, or through the urbanized suburbs that need it the most. Growth in the supply of stations and adjacent land is key, but it's a ground game. A couple of yards at a time.

Another way to make new, transitoriented land is to stack existing land: to create density. This is the most hopeful avenue for planners, who have advanced the cause of higher density at sites close to transit with courage, persistence, and the support of motivated developers.

But Manhattan-style density is not politically or physically available around many rail stops in the country. Take a look at the leafy suburbs along Chicago's lakeshore, Philadelphia's Main Line, or New York's Metro North, and you sense the political challenge in creating transit zones through intense rezonings in these settings.

Nationally, the average commuter rail trip is more than 23 miles, usually from distant suburb to city. Land for higher-density projects on these long commuter lines does appear at times, but it's often in places where people moved generations ago to avoid such urban-style living.