A man holding his mobile phone crosses a street with his luggage in Tokyo, Japan, March 18, 2015. Reuters

Pedestrian fatalities in the U.S. hit a 25-year high in 2016, and researchers think that smartphones might be the cause of the spike.

There were nearly 6,000 pedestrians killed in 2016, according to preliminary data released Thursday by the Governors Highway Safety Association. That figure represented an 11 percent increase over 2015, when 5,376 pedestrians were killed, and was the largest single year increase in 40 years. The rising tide of roadside deaths was even more pronounced since the start of the decade: between 2010 and 2015, pedestrian deaths have increased 25 percent. Researchers said there doesn't appear to be any other variable besides increased smartphone usage, which can lead to distracted walking and driving, that could explain the spike.

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"It's the only factor that that seems to indicate a dramatic change in how people behave," Richard Retting, author of the report and safety director for Sam Schwartz Transportation Consultants, told the New York Times.

According to a 2015 Pew Research survey, smartphone ownership nearly doubled between 2011 and 2015, growing from 35 percent of U.S. adults to 68 percent of U.S. adults over that period.

While traffic deaths were up overall, the deaths of pedestrians were increasing at a higher rate than deaths of vehicle occupants, suggesting that distracted drivers could be more dangerous to pedestrians than other drivers — or that pedestrians are putting themselves in harm's way more often. Pedestrian deaths rose as a total percentage of traffic deaths over the last decade, from 11 percent in 2006 to 15 percent in 2015.

Walking and miles driven have ticked up in recent years, but not enough to explain the rise in pedestrian deaths, Retting told the Times. The National Safety Council estimated there were 40,000 traffic deaths in 2016, the highest total since 2007.

While recent increases in traffic and pedestrian deaths may be troubling, American roadways are much safer than they once were, due to safer vehicles and more stringent traffic laws. In 1925, for example, vehicle crashes took the lives of 20,000 people — half of the 2015 total — in spite of the fact that American drivers drove more than 25 times the number of miles in 2015 as they did in 1925.

However, compared to other economically developed countries, the U.S. has the highest rate of crash deaths, according to a 2016 study. That is more than twice the rate of Sweden and the U.K.

The Governors Highway Safety Association report was based on figures collected from all 50 states and the District of Columbia in the first half of 2016 that were used to project totals for the entire year.