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Here's a scary thought for automakers celebrating the return of the auto boom: It's already over.

"Trends suggest that motorization in the U.S. might have reached a peak several years ago," Michael Sivak writes in an analysis for the University of Michigan's Transportation Research institute.

He's at the university's Sustainable Worldwide Transportation unit, and has been studying motor-vehicle density and use in the U.S. The latest report is called "Households Without a Light-duty Vehicle" (light-duty vehicles means cars, SUVs and standard pickups).

"Recent studies have shown that -- per person, per driver, and per household -- we now have fewer light-duty vehicles, we drive each of them less, and we consume less fuel than in the past," Sivak writes. "The recent increase in the proportion of households without a vehicle provides additional support for the hypothesis that motorization in the U.S. peaked during the previous decade."

Other analysts agree there's a slump in vehicle ownership, but are less likely to see as an anti-car shift in social mores or demographics that to see it as the fallout from the Great Recession.

The proportion of "second or third vehicles in a given household has dropped. People are consolidating their needs into fewer vehicles to cut vehicle costs," said Karl Brauer, senior analyst at Kelley Blue Book. But, he said, "population growth means total car sales will to rise even if registrations per household drop."

The number of vehicles on the road in mid-2013 was 247.9 million, highest since 2008, according to an Experian Automotive report late last year. Experian said that slipped slightly to 246.9 million the third quarter of last year, but that still was up from the same quarter a year earlier.

Sivak's main observations:

• In 2012, 9.2% of U.S. households were without a vehicle, compared to 8.7% in 2007.

• In six of the 30 largest U.S. cities more than 30% of households do not have a vehicle.

• From 2007 to 2012, there was an increase in the proportion of households without a

vehicle in 21 of the 30 cities.

Such statistics play into a common notion that young people today don't covet cars the way their parents did -- and are happy to ride bikes, use car-sharing and get around in other ways.

But that might not be so, said Lacey Plache, chief economist at Edmunds.com.

Her data show that once jobs are easier to get, the 18-to-34 age group will dive into the work world, "get good jobs and form households. Once they get out there and form their own households, then the average (car ownership numbers) go up."

"The number of vehicles on the road will continue to grow," Plache said.

Sivak's conclusion also fits neatly with the notion that big cities -- where most people live -- offer better public transportation, trimming the need for individual vehicle ownership.

But that, too, might not be the full story, Plache said. "You get to a point in life where a car becomes very necessary. There's a lot of talk about these car-share arrangements. But the projections for miles traveled for those is very small."

And at least for awhile, new vehicle sales will grow simply because people who nursed old vehicles through the recession now finally are dumping them for new ones. The average age of a vehicle on the road still is about 11 years, according to industry and analysts' calculations.

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