I had a telling conversation with an old friend several months ago, a devoted environmentalist who's a community college biology teacher living south of San Francisco in a pleasant small town abutting the Pacific. I don't recall how it came up, but she declared, "We've just got to get more people out of their cars." Then came a pregnant pause, followed by her admission that of course, because of where they lived and worked and their packed daily schedules, she and her husband drove themselves and their children everywhere.

I've been thinking about this lately because, well, the roads are still chock full of cars and trucks, and despite an uptick in transit and bicycle use, traffic is still congested here in metro Seattle, and metro regions nationwide. Meanwhile, U.S. surface transportation needs will require some $12.5 trillion (yes, with a "t") over the next 50 years, according to a landmark federal report issued this year. But the way we fund such projects is broken, relying too much on dwindling by-the-gallon gas taxes due to improved fuel efficiency, and ever more difficult local and regional sales tax hikes.

The historical trends show whopping increases in U.S. miles driven and gasoline supplied. We've gone from 2 million barrels of gas a day in the 1950s to more than 9 million per day by 2007, the U.S. Energy Information Administration reports. The U.S. Bureau of Transportation Statistics reports that U.S. vehicle miles traveled (VMT) multiplied more than fourfold from 1960 to three trillion in 2006. Though the term "highway" is sometimes attached to VMT, they are estimated monthly for all U.S. roads and streets, drawing from data gathered at 4,000 continuous traffic counting locations.

What of the future?

BTS projects that VMT will grow by more than half the current level to 4.7 trillion in 2030, while U.S. population grows about 23 percent from 2005 to 2030. In Washington state, annual VMT nearly doubled between 1980 and 2007, and is projected to rise another 54 percent by 2030.

In the four core counties of metro Puget Sound, daily VMT has more than doubled between 1980 and 2007.

During the oil and gas price run-up earlier this year, drawing considerable media attention were marginal decreases, of a few percentage points only, in monthly and calendar year-to-date U.S. VMT compared to a year ago. Even a slight dip in VMT draws notice in a time when some celebrate the end of suburbia and advocate "carectomies."

One can hope. These days, it seems that every seminar addressing surface transportation and every green "visioning" session includes earnest discussion of how to "reduce vehicle miles traveled." To the skeptic, the imperative sounds like one of those wishful commands sported on the seven-bumper-stickered Subaru Outbacks endemic to Seattle, like "World Peace Now," or "End Poverty." It's an appealing idea, sure. But the devil is in the details.

The infrastructure crumbles

In the meantime, there's still a pressing need to deal with roadway and bridge wear and tear, and increased congestion resulting from exponential VMT growth during a post-Interstate-building era when transportation investment chronically lagged. One reminder comes via veteran Chicago Tribune transportation reporter Jon Hilkevitch, who this month wrote that despite a five percent regional drop in VMT, traffic congestion there has remained high. One big reason:

Roadways were already so badly saturated with traffic before the recent spikes in fuel prices that the decline in miles traveled hasn't significantly loosened the gridlock.

Most daily trips in metro regions actually aren't to and from work, a point often overlooked. But many of those trips by their nature are less likely to involve transit. If you're going to Costco or Lowe's or Target, to your in-laws in Olympia or friends in Lynnwood, to curriculum night at your kid's school across town, or your cottage on Whidbey Island, you're most likely to be driving. Of total daily trips in the four-county core of the Puget Sound region, only 4 percent were via scheduled public transit, according to a 2006 Puget Sound Regional Council survey (second paragraph of p. E-6, here).

Work-related travel is somewhat more predictable, and there's more room, potentially, to change behavior and actually get some people out of their cars, some of the time. But progress there had been scant. The BTS also reports that — based on federal surveys and Census data — between 1989 and 2006 the percentage of U.S. workers for whom the principal means of transport to work was solo driving remained at 76. Those workers usually carpooling declined very slightly, to 10 percent of the workforce over the same 17-year stretch, and those usually taking public transportation decreased from 4.6 percent to 4.3 percent. Walking, biking, taxi, and "other" principal means of conveyance to work grew from a combined 4.7 percent of the workforce in 1989 to 5 percent in 2006, while telecommuting increased from 2.6 percent to 3.9 percent.

Numbers for 2007 and 2008 will likely show some decrease in solo driving to work, and a shade more transit use nationally, but without drawing up a whole new landscape, prospects remain iffy for reducing VMT or merely curtailing its growth.

As politically unpalatable as it seems now — and that would be "very" — some experts believe within a few decades we'll be tolling not just managed highway lanes with time- or congestion-related variable fees but tolling every mile traveled, via GPS devices planted on most if not all vehicles. "VMT tolling" or "mileage fees" have already been studied in Puget Sound and Oregon, and imposed on heavy trucks in Germany. This month, the Atlanta Regional Commission mused publicly about the unsustainability of the federal gas tax and the attractiveness of mileage fees. The Atlanta Journal-Constitution reported:

The board of the Atlanta Regional Commission is studying the idea of eventually dropping the federal gas tax, the main source of transportation funding, as it looks for "sustainable" transportation funding. The gas tax doesn't rise with inflation and gets weaker every year. The ARC, metro Atlanta's planning agency, hasn't approved a final statement on the issue and has no authority to implement it. The agency is giving its recommendations to Congress, as it begins to look toward renewing the multiyear federal transportation funding law.

The gas tax is charged as cents-per-gallon instead of cents-per-dollar, so the same size tank always reaps the same amount of money in taxes, no matter how much the price of gas goes up. In addition, as people get more fuel-efficient cars, they use less gas, and so pay less gas tax. The ARC suggests more research on one of the more talked-about ideas, an odometer charge, or vehicle miles traveled. Such a charge would tax drivers by the amount of miles they drive. The idea is for drivers to pay for the wear they put on the roads. Depending on how sophisticated the tracking is, it could send the tax paid directly to the jurisdictions whose roads the driver uses. To avoid getting weaker every year, as the gas tax does, it would have to be designed to rise with inflation.