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WASHINGTON, D.C. (NEXSTAR) — New numbers out from the federal government show fewer Americans are dying in traffic accidents, but officials aren’t sure exactly why that is.

According to the National Highway Transportation Safety Administration, traffic deaths in 2018 dropped for the second year in a row, with more than 900 fewer deaths compared to 2017.

“We can’t attribute this to any one set of factors,” said NHTSA Acting Administrator James Owens. “We simply don’t know. We are digging into the data.”

According to the numbers from NHTSA, deaths involving children, alcohol, speeding, and motorcycles all fell, but the specific reasons why are elusive.

“We know that human behavior, human error, accounts for that vast majority of serious car crashes,” Owens said.

And in the absence of specific causes, NHTSA is relying on drivers to keep the positive trend moving.

“If we could get drivers to stop doing three things, we could save about half the lives on our roads every year,” Owens said.

Those three factors, the NHTSA said, are wearing a seatbelt, not driving impaired, and not speeding.

But even though traffic fatalities were down, data shows the number of pedestrians and bicyclists killed actually went up.

“We think as researchers start digging into this data, we’re going to get a lot more answers,” Owens said.

But the overall numbers are still improving, and so far in 2019, officials said traffic deaths are falling at an even higher rate.

“And we’re very hopeful that we’re going to see further reductions in the future,” Owens said.