Americans set a new record for driving in 2015 with 3.15 trillion miles traveled.

Americans drove more than ever last year, clocking a record 3.148 trillion miles behind the wheel, according to new data from the Federal Highway Administration – but it appears that they’re also dying in larger numbers, with preliminary crash data showing a sharp reversal of a decade-long downward trend in highway fatalities.

The jump in the number of miles Americans are driving comes at a time of increased economic activity, particularly in the auto industry, which sold a record 17.5 million vehicles last year, a 5.7% increase over 2014. And while many observers had expected the numbers to flatten out this year, demand has maintained a torrid pace.

Exactly why Americans are driving more is unclear, however, Federal Highway Administration spokesman Doug Hecox telling the Minneapolis Star Tribune, “We just know there are more cars out there and they’re going farther.”

The latest available data suggest there are about 260 million registered vehicles on U.S. roads. That figure is growing not only because of record car sales, but also because owners are keeping them on the road longer than ever, an average 11.5 years, according to Detroit-based tracking firm Polk.

The number of licensed drivers, meanwhile, has grown sharply, from 163 million in 1988 to 210 million at the beginning of the decade. However, the pace of that growth has slowed, as the University of Michigan Transportation Research Institute finds Millennials are getting licensed at a far slower pace than their Baby Boomer parents and grandparents.

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Millennials are also more likely than prior generations to move back into central cities where, even if licensed, they are less likely to drive or own a car.

But while the Great Recession put a temporary clampdown on the century-long growth of suburban sprawl, the latest government housing data show that Americans are once again embracing the idea of a dream house in the suburbs, and the level of commuting this requires.

But there are other factors that are also getting Americans back in their cars again. Historically low gas prices are clearly playing a part. Nationwide, pump prices plunged to well below $2.00 a gallon early this year. The AAA reports that the recent rebound in crude oil prices has driven the average figure up to around $2.04 for a gallon of self-service regular.

But that remains well below the record $4 a gallon seen less than two years ago and, adjusted for inflation, it’s actually a lower figure than what Americans were paying for gas before the twin Mideast oil shocks of the 1970s.

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Whether the trend towards driving more will continue is far from certain. Researchers are betting that as Millennials age, many will embrace the classic American dream and move back out to the suburbs. And another recent study by AutoTrader found that members of the next generation, Gen-Z, are as likely as Boomers to want to get licensed and own a car.

On the other hand, emerging alternatives such as ride-sharing services like Uber and Lyft could see fewer Americans actually buying cars, if not traveling less.

One trend that worries observers is the sudden surge in highway deaths. The figure fell to just over 32,000 in 2014, a 40% decline from the peak years of the 1970s. But preliminary data released by the National Highway Traffic Safety revealed the death toll rose about 8% during the first nine months of 2015. And states like Michigan are reporting still further increases this year.

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The precise cause is unclear, but most experts believe that the surge in the number of miles Americans are driving has been a major factor behind the rise in highway deaths.