It was a great year for auto sales in 2014, but a new six-part study says that American interest in cars may be on the decline.

Looking at the number of cars and light trucks registered in the U.S. from 1984 to 2011, Dr. Michael Sivak of the University of Michigan Transportation Research Institute found that vehicle registrations peaked at 236.4 million in 2008 before falling to 233.8 million in 2011, the last year for which data was available.

As reported by Edmunds.com, the study revealed that the number of miles per driver declined as well, rising to a peak in 2004 before falling. In the third research phase, Sivak discovered that the amount of fuel used per licensed driver and per vehicle also reached a high in 2003 or 2004 and then dropped.

The researcher notes in the study that "the onset of the downward trends--of the number of vehicles on the road, miles driven, fuel consumed and vehicles owned--precedes the beginning of the recession in 2008," Edmunds reported.

Another installment in the study showed an increase in the proportion of households without a vehicle in 21 of 30 major American cities.

The falling number of vehicles could be a long-term trend.

Sivak wrote that "the reductions in these rates likely reflect fundamental, noneconomic changes in society (such as increased telecommuting, increased use of public transportation, increased urbanization of the population, and changes in the age composition of drivers), with economic factors being contributing factors only. Therefore, these maxima have a reasonable chance of being long-term peaks as well."

Some industry analysts say that young people are less interested in owning cars and are turning to public transportation and app-based car services instead.

Auto sales in the United States are expected to reach 17 million next year for the first time since 2001, Reuters reported. Sales for December 2014 will be 10.4 percent higher year over year.

This year's numbers jumped 5.8 percent from 2013 to 16.5 million U.S. auto sales. 

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