The first is traffic congestion and its limiting effect on growth. Once a city or metropolitan region reaches a certain size, it needs a rapid transit system to prevent crippling traffic congestion, especially during the morning and evening peak hours. Heavy rail systems like the Washington, DC Metro or New York's Subway carry far more people per hour than expressways can. And without transit, a city and its metropolitan region will face limits to growth. That's why we see cities like Houston, Dallas, Salt Lake City, Seattle, and Charlotte building new rapid transit systems or expanding their existing systems. This is also why local chambers of commerce are often the biggest pro-transit boosters. They know that without transit, a city becomes clogged in traffic, productivity drops, and businesses begin looking elsewhere.

An article in the Atlanta Journal-Constitution made this clear:

"I continue to be frustrated that we can't seem to move in that direction," said Sam Olens, chairman of the Atlanta Regional Commission and the Cobb County commission. "We're losing our competitive advantage." Two years ago, the Metro Atlanta Chamber of Commerce invited reporters to hear officials from Phoenix and other cities talk about their new transportation initiatives. The message was clear: Atlanta and Georgia could be left in the dust.

Cars also take up a lot of space. Expressways and their entrance ramps take up tremendous amounts of land. When cities get bigger, and land becomes more expensive, this represents tremendous opportunity costs; this land could otherwise be available for housing, offices, or parks. And then there is the parking, acres and acres of it. This blog recently imagined what New York City would look like without its Subway system. Without the Subway, New York City would need 76 Brooklyn Bridges to carry people into the Central Business District, and 3.8 square miles of parking to store their cars once they got there.

You could argue that since New York is the larges city in the country it is a special case. But we're an increasingly urban country. Our cities are growing quickly. One day Houston will look more like Brooklyn than its current sprawling form. Already its small single-family homes are being replaced by row homes. Will Houston ever be able to build enough expressways to handle its growing population? If so, will there be any room left for people?

Without transit, we can't significantly increase density in our metropolitan cores. And without increased density and infill development, metropolitan regions will have no choice but to continue growing outward. The result will be ever-longer commutes for households. More workers are traveling over an hour to work every day as new homes are built further and further from jobs. People want and need to live in metropolitan areas; 68 percent of Americans choose to live within 1.8 percent of the country's land area. In order to accommodate them, and to plan for an increasingly urban nation, we will need to continue to invest in our mass transit systems.