WASHINGTON -- Traffic deaths surged last year as drivers racked up more miles behind the wheel than ever before, a result of an improved economy and lower gas prices, according to preliminary government data released Friday.

Fatalities rose 7.7 percent to 35,200 in 2015, said the National Highway Traffic Safety Administration. Last year was the deadliest driving year since 2008, when 37,423 people were killed. It was also the year in which American drove 3.1 trillion miles, more than ever before.

The Northwest region, including Oregon, Washington, Idaho, Montana and Alaska, saw the nation's biggest increase in fatalities with a 20 percent jump.

The information comes as tens of millions of Americans were hitting the road for the Fourth of July holiday.

Historical data show that after peaking in the 1970s, traffic deaths have fluctuated quite a bit while generally trending downward, according to the Insurance Institute for Highway Safety. Large dips in deaths have corresponded to shocks to the economy: the oil embargo of the mid-1970s, the recessions of the early 1980s and early 1990s and the more recent downturn that began with the subprime mortgage crisis.

"It's not just that Americans drive more miles when the economy improves, it's the kind of miles they drive," said Russ Rader, a spokesman for the insurance institute. "What comes back after a recession is the optional driving that's riskier, like going out on the weekends or taking long trips -- different driving than the daily commute."

The national average price of gas in 2015 was $2.40 per gallon, which was the second cheapest annual average of the past decade, according to AAA. It was about 94 cents per gallon less than the annual average in 2014, which also saw the lowest number of traffic deaths -- 32,675 -- since Harry Truman was president.

"The upticks (in deaths) we're seeing correlate to lower fuel prices, but we don't want to give ourselves that excuse so we are digging into different areas where we can have an impact on this," Transportation Secretary Anthony Foxx told reporters earlier this week. The department, which includes NHTSA, is looking at how advances in automotive technology can reduce the death toll, he said. NHTSA's revamping last year of its safety rating system for new cars to include automated emergency braking technologies may help, he said.

Motorcycle deaths are another factor that may be driving up fatalities over, said Jonathan Adkins, executive director of the Governors Highway Safety Association, which represents state highway safety offices. The association estimates motorcycle deaths were up 10 percent last year, topping 5,000 killed for the first time since 2008.

"Motorcyclists are a bigger and bigger percentage of deaths each year," Adkins said.

Just because the state of the economy is a significant factor in the rise and fall of traffic deaths, "it doesn't mean that policymakers are powerless," Rader said. If people followed a few simple countermeasures like driving more slowly, buckling up and wearing a helmet when riding a motorcycle, he said, "the increases we're seeing now wouldn't be as large."

-- The Associated Press