In the modern world, we cherish our freedom and individuality. And, as automobile advertisers have long understood, few experiences make us feel more liberated than a fast ride with the top down.

To be modern is to be mobile. Our economy depends on the free and rapid circulation of people and goods, and we have invented transportation technologies to suit our needs. First the railroads moved people and goods at previously unimaginable speeds, while steamships circled the globe. Then, in the 20th century, airplanes moved us even faster.

For most people, though, mobility means automobiles. Cars freed rural people from their isolation, and they gave city dwellers access to the countryside. The middle classes take their cars for granted, while the world's poor aspire to car ownership as a token and a tool of advancement.

But is our modern mobility sustainable? We are facing an energy crisis, a climate crisis, and an economic crisis - and perhaps a mobility crisis as well.

Sometime during the past two years, according to United Nations estimates, the world's population became mostly urban. For the first time, a majority of us now live in cities, and that majority will grow rapidly. But urban life poses a challenge to our automotive mobility. In cities, cars offer easy mobility - but only when traffic isn't too bad. They also free drivers from the delays and tribulations of buses, trains and sidewalks. In other words, a car furnishes a steel cocoon that shields motorists from their fellow citizens.

That protection comes at a price, if not a loss of civility, then certainly of urban mobility, as cars clog the roads. Individuals still find it worthwhile to drive through the city. But for the community as a whole, making enough room for all the cars means devoting a large part of the city to a costly network of highways and parking lots - which still can't keep up with the growing fleet of cars.

Amid road construction and honking horns, the vaunted freedom of the open road has long since vanished in the rear-view mirror. If we continue to fill our cities - not just London and Los Angeles, but also Mumbai and Shanghai - with cars, we will be left with little mobility and barely functional cities.

Which brings us to the state of the auto industry. Although the crisis of the American automobile manufacturers has many causes, its resolution may founder on the crisis of mobility.

The United States has always been the model of a mobile society, where people are willing to pull up stakes and go someplace new - if not to a new territory out west, then at least to a distant suburb, a long drive away from their old home. The US became the envy of the world not least because it was the first country where ordinary people owned cars, and it is still the land where people drive really big cars. It led the way in rebuilding its cities around cars and highways.

Even as the automobile business went global, the US car companies remained a breed apart. Although Ford and General Motors build small cars in Europe, they made their money at home by persuading urban dwellers to buy gas-guzzling behemoths that promised mastery of the open road. Detroit owes its century of remarkable success to the carmakers' skill in packaging practical mobility with improbable fantasy.