Greg Fischer, the mayor of Louisville, takes out a piece of paper and draws a map that might give the US motor industry cause for concern. It shows the chain of parks that is taking shape round the outskirts of his city, the largest in Kentucky, and the 100-mile orbital route that will link them. Mr Fischer, whose city is criss-crossed by a series of elevated highways and wide, four-lane avenues, is not planning another road, however. Instead the mayor is planning a cycle route to link up the parks – reflecting, he says, growing demands from residents for alternatives to driving.

Mr Fischer’s effort is the kind of initiative that would once have been unthinkable in a mid-western state such as Kentucky, where transport planning has centred almost exclusively around the car for decades. Yet the push by the mayor, who envisages 28 miles of bike lanes in the city centre to encourage car-less apartment-dwellers to move in, is far from unusual. Similar efforts are under way in New York, Boston, Washington DC, Chicago and San Francisco.

Along with Dallas, Denver and other large US cities, Mr Fischer is looking to connect the city to its sprawling suburbs with a rapid transit system.

The efforts infuriate some observers, including Sean McAlinden, chief economist at the University of Michigan’s Centre for Automotive Research, who say they are doomed to fail. They insist America is fundamentally car-dependent and likely to remain so.

“I really don’t know what’s going on here,” Mr McAlinden says. “I believe that once there’s …a turnround in the fortunes of all those in the labour market, they will once again rely on motor vehicles as their primary source of transportation.”

Yet there are increasing indications that the century-long love affair between Americans and their cars – and the suburban lifestyle it produced – might be cooling. Executives at the big carmakers are watching the trends with a wary eye.

Some of the evidence is statistical. Car ownership levels are declining and so is the average distance each American drives in a year. More worrying for carmakers, the declines are particularly steep among the younger, so-called millennial generation.

Less scientifically, there are anecdotes from all over the US – including sprawling sunbelt cities such as Fort Worth and Atlanta – of inner-city brownfield sites filling up with apartment buildings. With few car parking spaces but easy access by bike or on foot to local amenities, these developments signal a reversal of decades of growth away from the city centre. Younger professionals appear to prefer the idea of living in a converted inner-city warehouse to enduring the hours of car commuting.

Few believe the evidence points to an imminent divorce between America’s still heavily car-dependent cities and the motor car. But a frosty estrangement is a possibility, says Michael Tamor, a senior executive handling sustainability questions for Ford, the second-biggest US carmaker.

Such fears have prompted both Ford and General Motors, the biggest American carmaker, to discuss publicly whether their future might be as “mobility providers”. This could mean leasing cars for short periods or making electric bikes rather than just building and selling cars.

“We’re trying to predict where things are going to go,” Mr Tamor says. “Whether or not the automobile remains viable [in future cities], it doesn’t mean it will remain the most favoured transportation mode.”

Mr Fischer recognises that cars will remain vital – hence his support for a $1.4bn bridge across the Ohio River. But within the city itself the priority is to make neighbourhoods more “bikeable” and “walkable”.

“There’s a lot of experimentation going on,” he says.

For millennials, the car is different. It’s not the iconic freedom machine that it might have been for a baby-boomer

The question for policy makers and carmakers is how big and how lasting these trends are. Many believe that urban sprawl is a fact of American life and will resume once the economy strengthens. But one of the strongest pieces of evidence that a structural shift is under way is that vehicle ownership rates per household began to fall even before the economic downturn. The rate had reached a peak of 2.05 in 2006, but was back down to 1.95 in 2011, says Michael Sivak, director of the sustainable transportation programme at the University of Michigan.

The average number of miles driven per person in the US has also been declining steadily since 2004, when it reached 9,314 miles. The figure for 2011 was 8,494 miles, a level last seen in 1996. The decline has been so sharp that in spite of the country’s continued population growth, there have been overall declines in some years in the aggregate number of miles driven.

“I think there’s some indication we’re reaching some kind of saturation,” Mr Sivak says.

The question is why. One theory holds that the ageing baby boomer generation is gradually leaving the workforce and, in some cases, retiring to downtown apartments or other places where there is less need to drive.

But the declines are sharpest among the youngest drivers. The US National Household Travel Survey, conducted by the Federal Highway Administration, found that average vehicle miles travelled by 16- to 34-year-olds fell 23 per cent – to 7,900 miles – per capita between 2001 and 2009. Between 2000 and 2010 the number of US 14- to 34-year-olds without a driving licence increased from 21 to 26 per cent.

While earlier generations felt liberation behind the wheel of a car, young people may find a sense of freedom through their smartphones and tablets, car company executives say.

“For millennials, the role of a car is different,” says Catherine Lovazzano, senior manager for consumer trends at Chrysler, the number three US carmaker. “It’s not the iconic freedom machine that it might have been for a baby-boomer.”

The millennials are also playing a role in the shift to the cities, as downtown Louisville shows. They do not appear to have the same aspiration to a “borderline rural, suburban lifestyle” as their parents, Mr Tamor says. “That just doesn’t work for some people.”

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But the scale of the shift into cities and away from cars will only be clear as millennials recover from the effects of the recession. Alan Pisarski, author of the respected “Commuting in America” reports, is sceptical that driving is in long-term decline. He notes that unemployment has hit millennials far harder than any other age group. They also tend to be loaded with high levels of student debt and face prohibitively high car insurance costs.

“If you look at the kinds of financial problems that not only the young people but their parents have to manage in many cases, we should not be surprised that they’re riding buses and living through their cellphones,” Mr Pisarski says. “One could certainly make the case that the economic strictures are a fairly adequate explanation for what’s going on.”

The decisive point may come when large numbers of millennials start to settle down and have children, the life-changing event that sent many preceding generations scuttling for the spacious gardens, good schools and large cars of the suburbs.

“The question of how you get around in the urban environment with little kids really starts to change the script,” Ms Lovazzano says. “It’s yet to be seen whether or not millennials will leave the city.”

Yet it looks unlikely that a generation that has shown itself so much less prone to driving than its predecessors will catch up with previous generations. Phineas Baxandall, a senior analyst for US PIRG, a federation of research groups, concedes that some millennials will move to the suburbs and drive more. The question is how their driving rates will compare with those of their parents.

“That’s where the evidence suggests yes, they will drive less, regardless of what the economy does,” Mr Baxandall says. “There’s something more going on.”

Mr Baxandall co-wrote a report published in May outlining the evidence that car use is declining – and calling for policy makers to rethink planning projections which assume driving rates will continue to grow as they did in the driving boom after the second world war. “The report was calling on transportation planners to at least put a question mark after the presumption that vehicle miles travelled per person will continue increasing,” Mr Baxandall says.

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Even some car company executives appear to be thinking on similar lines. Wade Bryant, advanced design manager for General Motors, says the company may have to re-examine its approach to selling cars.

GM, like some of its rivals, is enthusiastic about its relationships with new sharing services such as Zipcar, which offer members by-the-hour access to cars parked in their neighbourhoods when they need them. Such services are likely to keep demanding regular supplies of new vehicles.

“It’s not just a car that we sell to someone and then welcome them back in seven years to buy another one,” Mr Bryant says. “It’s how we help someone to get around. It may not be all just automobiles.”

It may even be that, although fewer vehicles are sold per person, US population growth ultimately pushes aggregate demand up above the record 16m-17m vehicle per year level that it reached in the middle of the past decade. After a strong recovery from a plunge to annual sales of just over 10m vehicles in 2009, the industry is now selling about 15m cars a year.

“As the economy improves, most likely we’re going to see the previous peak surpassed, partly because there will be more people around,” Mr Sivak says.

Yet that new record looks set to be reached more slowly than it would have been during the postwar driving boom. Many observers also expect future cars to occupy roads modified along the lines Mr Fischer is pursuing in Louisville. They will carry a far higher proportion of pedestrians, cyclists and public transport passengers than in the recent past.

Mr Tamor nevertheless cautions that, while there may be significant changes under way, the shift might at first appear barely perceptible.

“There could be very real and very powerful forces at work but you’re steering a very large ship,” Mr Tamor says. “The changes may be slow.”

Science fiction vision holds no allure for drivers

For decades it has been a standard trope of science fiction to picture the cars of the future as small models humming efficiently around a clean utopia.

But for the staff at America’s big car companies, it is far from clear that future consumers really will crave anything radically different from the mixture of passenger cars, sports utility vehicles and pick-up trucks that they drive today.

Nor, according to Michael Tamor, a senior executive dealing with sustainability at Ford, will they necessarily be that small.

“The thing that a lot of people forget is that if you’re going to have one automobile, it really has to do all the things that you do in an automobile,” Mr Tamor says.

“Even if you live in a high-density area, if you need it to take a family of five on a long trip once a year, you’re constrained in how small it can be.”

Alan Pisarski, author of the Commuting in America reports, points out that the cars might also be used for longer commutes.

Even if they live in densely populated, walkable areas, growing specialisation in some careers might push future urban dwellers to commute to other, far-flung areas to work. That is particularly the case because ever-greater numbers of couples have two careers.

“The chances of both of them being able to live near to work are close to zero,” Mr Pisarski says.

The pattern of development dominant after the second world war – and particularly following the urban unrest of the 1960s and 1970s – was characterised by wealthier city dwellers fleeing to the suburbs while poorer residents moved into the inner city.

But radical changes in urban living patterns present a big challenge for carmakers.

Any drift by wealthier residents and young professionals back into inner cities, Mr Tamor points out, could push the less well-off into car-dependent suburbs.

Poorer people’s car demand, however, would affect manufacturers only slowly because few would be likely to buy new cars, Mr Tamor says.