Nathan Bomey

USA TODAY

U.S. traffic deaths jumped 7.7% in 2015, marking the deadliest year on the road since 2008, auto safety regulators reported Friday.

An estimated 35,200 people died in vehicle crashes in 2015, just a year after roadway deaths plunged to a modern-era low of 32,675 in 2014, according to the National Highway Traffic Safety Administration.

Potential explanations for the uptick include an increase in driving fueled by low gasoline prices and the threat of distracted driving enabled by mobile devices.

But NHTSA regulators said Friday that they are currently unsure exactly why traffic fatalities reversed their steady decline in 2015.

“Every American should be able to drive, ride or walk to their destination safely, every time,” U.S. Transportation Secretary Anthony Foxx said in a statement. “We are analyzing the data to determine what factors contributed to the increase in fatalities and at the same time, we are aggressively testing new safety technologies, new ways to improve driver behavior, and new ways to analyze the data we have, as we work with the entire road safety community to take this challenge head-on.”

The jump in fatalities for the year was widely expected after earlier figures showed a 9.3% uptick through the first nine months of 2015 to about 26,000.

The last time roadway deaths for a full calendar year hit a higher mark was 2008, when 37,423 people were killed.

The report also comes as Foxx and NHTSA administrator Mark Rosekind have accelerated their push for self-driving vehicles, which they believe can ultimately eliminate roadway deaths altogether.

But concerns about self-driving cars swirled anew this week following Thursday's disclosure that a Tesla Model S driver who had enabled the vehicle's Autopilot feature died following a collision with a truck. Tesla says it was the first death in the crash of a Model S in 130 million miles of driving in which the self-driving feature was engaged. That compares with a death every 94 million miles in driving overall, the automaker said in a blog post.

"Self-driving vehicles hold much promise for improving road safety," says Michelle Krebs, senior analyst at AutoTrader, in a statement, "but more work is needed with the technology, regulations and consumer confidence, which could be shaken by accidents like this.”

Follow USA TODAY reporter Nathan Bomey on Twitter @NathanBomey.