The American Public Transportation Association tells us that Americans took 10.3 billion trips on public transportation in 2007 -- the most since 1957. (Thanks to Environmental Economics for the link.)

There are reasons to consider this good news, if only because it proves that higher gas prices do eventually result in behavior changes. The truckers and farmers lamenting "that we just can't get along without diesel" in a Tuesday New York Times story on rising fuel costs will find ways to minimize consumption, the higher prices get. And the APTA is right to cite the new figures while calling for more federal support of public transit systems.

But I was most struck by the news that fifty years ago American reliance on public transportation was so high. The U.S. population has almost doubled since then, from around 172 million to today's 302 million. So on a public-transport-ride per capita basis, the U.S. is still nowhere near the heights reached in the 1950s.

So what happened? Here's one clue. In 1956, Dwight Eisenhower signed into law the Federal Highway Aid Act, which authorized the creation of the United States Interstate Highway System.

Isn't that a lovely synchronicity? The Interstate system is born, and almost immediately, public transportation in the United States embarks on a long decline.