This week marks the 50th anniversary of the National Defense Highway System, America's multi-decade, multi-billion-dollar building spree, initiated by President Eisenhower, resulting in this nation's massive network of freeways and highways: 47,000 miles worth, enough asphalt to coat the world twice. The NDHS was sold using a Cold War rationale: when the Russian and Chinese armies approach our shores, we need to get American troops and tanks there, pronto. This problematic explanation served as cover for the NDHS's other (and some would say, more realistic) purpose: to encourage high-speed auto and truck traffic; accelerated urban growth, worker migration, and commerce; and the expansion of the auto industry. The NDHS succeeded magnificently on all counts. And each factor increased the consumption of oil.

The NDHS' growth was paralleled by a succession of Federal aviation laws that resulted in building more airports and establishing the FAA's national flight control system.

These magnificent government projects produced equally magnificent subsidies for the auto and airline industries and tremendously ramped up Americans' mobility. They also contributed to the decline of mass transit, particularly buses, trolleys, and trains that didn't receive similarly robust subsidies. The same has happened overseas, except that in most countries, trains have survived as part of a larger social contract. Burgeoning business-travel, tourism and hospitality, and telecom industries have been built on the expectation that more and more nations will become mobile societies. And we've come to take for granted a certain ease of motion that is part and parcel of the experience of living in a modern industrial society.

Instead, we're headed for an Age of Immobility. Not because there are too many cars on the road or planes in the air. Of course, there are. Everyone living in a city has experienced insane levels of traffic congestion and aching, two-hour commutes. The automobile is no longer a net-gain way to travel -- and air travel in the 21st Century? Forget it!

The ultimate cause of the Age of Immobility is that we are reaching “Peak Oil” -- characterized by the Hubbert Curve -- when the world is producing all of the oil that can be easily gotten, without heroic exploration. From that day forward, our oil stores and production will begin an irreversible decline. Peak Oil, when it arrives, will slowly strangle not only ground transportation, but also movement through the air and on the seas. Easy mobility will become another memory from the Golden Age of Petroleum. There is no escape in turning coal into oil, as some recommend: the process is long, expensive, environmentally catastrophic, and ultimately, requires more energy than it produces. Burning more coal to make coal into oil would only multiply the offense to the Earth.

Some say that Peak Oil already is here. In any case, its arrival in the next decades is almost inevitable. What will it mean for most people in the world, including the industrial nations, to experience constant immobility?

Think back to earlier societies, when the only accessible workplaces were, for serfs, the farm; for traders, the village market; and for small producers, the small shop easily walked to. Most people walked. Relatively few people owned horses or oxen, the only sources of mobility other than walking. There was no petroleum-fueled transportation, so goods and services were acquired locally or not at all. Only the very rich could afford to journey on business or pleasure. The nobility in their coaches, running roughshod through Paris streets before the Revolution, trampling anyone in the way, is a movie cliche that's hard to forget -- because once it was real.

There were exceptions. Wooden ships used wind and coal for power; and trains for a few brief years burned wood or coal to generate steam; but in those early industrial days, there were vast forests for the taking, and "global warming," to which burning wood and coal contributes, was unknown. Today's steel ships and high-speed trains run on oil, as do the automobiles and aircraft with which they compete, or using electricity generated by burning oil. In our future, ships and trains may join cars, buses, trucks, and planes as relics of an earlier era. Little by little, our roads and airspace will get emptier as cars and planes first get smaller and more fuel efficient, then begin to disappear, except for those owned by large businesses and wealthy individuals. For a short while, these last few will command the highways and airways; then they, too, will be used only for special occasions or cease to operate entirely, starved for petroleum-based fuel. Note that this goes for electric cars, too, at least as they currently are powered: by electricity generated from burning oil and coal. (See Corey Powell's "Black Cloud," a review of Jeff Goodall's new Big Coal in the NYT Sunday Book Review.)

Our children may live in a very different world. Award-winning author Ursula Le Guin's allegorical Always Coming Home describes a future pastoral society where walking is the main means of mobility; the trains are powered by oxen. (In Le Guin's future, cars and planes ceased to exist millenia earlier. What happened to all of the “People With Their Heads on Backwards” -- meaning you and me -- is a mystery. Supply your own unhappy theory.) People take it easy. But also, their ambitions are turned completely inward, toward community, ritual, and the routine. There is beauty, but it is small, personal, unsensational. The Internet, now called “The City,” has become sentient. It's an oracle occasionally consulted, but mainly out of sight and out of mind. What can technology offer these people without synthetic energy, rooted in place, whose furthest journeys are from today's Napa Valley to the San Francisco Bay? Cellphones? Mobile Internet? iPods?

How quickly scenarios like Le Guin's can become reality. We get a preview every time a shock occurs to the world's oil supply: producer boycotts, a civil war in Nigeria, earthquake damage in Indonesia, politics in Venezuela, fighting or sabotage in distribution choke-points like the Caucasus or Kazakhstan -- wherever there's oil, there's crisis, and then prices rise. But we've learned to live with momentary burdens. It's the larger, more benign events, like the growth of China's and India's economies, that will create disastrous outcomes for the world's supply of petroleum and petroleum-dependent, mainstay industries including manufacturing, pharmaceuticals, agriculture, defense -- and transportation. Of course, a nuclear war anywhere in the Middle East would hasten the petroleum economy's demise.

What is the experience of being limited in range to the distance one can walk in a day? Or doing business in terms of mule-miles? What consequences has immobility for sustenance, health, education, commerce, and community? Will we all inhabit villages again, albeit most of us within what used to be integrated cities? Perhaps those living in deprived regions of the world, like Darfur, Afghanistan, or Chad, have something to teach us about our own futures.

Barring a miracle, like major governments truly committing resources for developing renewable fuels, or the invention of a device for turning discarded plastic into bio-safe liquid fuels -- still only hopeful visions -- our ability to easily circulate, a freedom we've taken for granted, is in trouble. With the trivial exception of mixed shopping/loft developments, however, you don't see many designers designing for mass immobility. The very notion is taboo.

Blogs: check out Matt Savinar's Peak Oil: Life After the Crash, for a picturesque portrait of post-Peak Oil civilization; and C. Peppard's Getting There, devoted to “Transportation for the Masses.”