Fewer Millennials seem to be falling in love with the automobile, studies show.

We’re coming up on the centennial of the moving assembly line, the seminal event that helped put America on wheels.  Today, there are more cars than licensed drivers in the U.S. – but a new study adds to growing concerns that America’s love affair with the automobile might be coming to an end.

There have been a number of recent studies that seem to support that argument, including research by the University of Michigan Transportation Research Institute, or UMTRI, that Millennials are generally waiting longer to get their licenses than prior generations – if they get them at all.

But a new UMTRI study raises even more flags, noting that the number of vehicles on U.S. roads peaked in 2008 and has been steadily declining since then.

That could be the result of a weak economy, concedes author Michael Sivak. “On the other hand,” he quickly cautions, the trend was already in motion even before 2008, the rate of growth slowing before the automotive market dipped into its worst downturn since the Great Depression. That just could “reflect other societal changes that influence the need for vehicles,” according to Sivak.

What’s certainly true is that Americans aren’t adding automobiles to the national fleet the way they did in decades past.  From 1901 to 1921, the number of cars on the road increased by a factor of 1,000.  It then took until 1968 to grow the total tenfold.  From 100 million vehicles in 1968, the number of light duty vehicles peaked at 236.4 million in 2008 noted the UMTRI study, and dipped to 233.8 million three years later.

(Millennials wait longer, are less likely, to get drivers’ licenses. Click Here for more.)

Other studies show that over the last decade – starting even before the recession, American drivers were putting off buying replacement vehicles.  That means the typical vehicle on the road today is about 11 years old, the highest figure in modern times.

There are other ways to look at what’s happening that only encourage those who believe that “motorization” has peaked.  Between 2008 and 2011, even as the number of vehicles on the road fell by 1.5%, the overall U.S. population grew by nearly 2.5%.

Indeed, the number of light-duty vehicles in the U.S. hasn’t nearly kept up with the growth of the population for quite some time, according to UMTRI’s data.

There have long been more cars than licensed drivers in the U.S. — as recently as 2006, 1.16 vehicles for every driver. By 2010, that was down to 1.10.

That’s in line with a separate study by CNW Marketing released last month that found the number of U.S. households that have gone “car-less” has grown from 5.7% in 1991 to 9.3% last year – and could soon top 10% unless the economic recovery spurs a sizable number of potential buyers to return to showrooms.

(Growing number of Americans go car-less. For more on that story, Click Here.)

The flip side is that those who have vehicles may have lots of vehicles – or more precisely, there were 1.84 cars per household in 1984. By 2011, that rose to 1.95.

The good news for the auto industry: those families who now have cars may need to replace several.  But there are a growing number of American households which simply don’t own vehicles and may never buy one.

Just what this all means for the future? Well, the Motorization study dodges coming to a hard conclusion, suggesting that the long-term future of America’s love affair for the automobile will likely depend on “which (of) the relevant social changes turn out to be permanent.”