Sheriff's deputies and the Boise Fire Department dive team stand by the wreckage of an SUV driven by a 40-year-old Boise woman that plunged off a cliff into the Lucky Peak Reservoir June 2, 2016, in Boise, Idaho. Divers recovered the bodies of two girls, ages 12 and 6, and the body of a 10-year-old boy from the vehicle. (Patrick Orr/AP)

In an age when cars are getting safer, traffic fatalities were up sharply in 2015, adding a disturbing data point in the nation’s long-term trend toward less deadly roads.

The National Highway Traffic Safety Administration said Friday that an estimated 35,200 people were killed on the road last year, up 7.7 percent from 32,675 in 2014.

An early analysis of the data showed spikes in deaths of bicyclists, pedestrians and motorcyclists, which were up 13, 10 and 9 percent, respectively. Crashes involving young drivers were up 10 percent, those involving rollovers in passenger cars were up 5 percent and those involving large trucks were up 4 percent, according to the NHTSA estimates.

Federal officials pointed to economic factors as a contributor.

“As the economy has improved and gas prices have fallen, more Americans are driving more miles,” NHTSA Administrator Mark Rosekind said in a statement. But he added that “94 percent of crashes can be tied back to a human choice or error,” so changing behaviors and promoting crash-prevention technology are crucial.

Another factor helping to push the death numbers higher is an increase in speed limits in recent years, according to a recent report by the Insurance Institute for Highway Safety.

The institute said that in 2013 alone, higher speed limits resulted in 1,900 additional deaths, canceling out the lives saved by air bags that year.

Responding to questions Thursday at the National Press Club, Christopher A. Hart, chairman of the National Transportation Safety Board, had a one-word answer when asked how to deal with the increase in traffic fatalities.

“Automation,” Hart said. “For 20 years, we’ve been pushing for something that is a collision-avoidance system. That’s step number one, an avoidance system that stops cars from hitting one another.”

Jonathan Adkins, executive director of the Governors Highway Safety Association, agreed with Hart’s assessment. But he added a cautionary note, pointing to Tesla’s announcement Thursday that one of its drivers was killed while driving a Tesla vehicle on “Autopilot.”

“As the tragic Tesla death reminds us, focus on the driver can’t be forgotten in the looming age of autonomous vehicles. Technology will help keep us safer on the road, but we are a long way from the day of truly self-driving vehicles,” he said.

While troubled by the rise in deaths, Adkins said there are proven tools to address the issue.

“They include strong laws coupled with highly-visible law enforcement and robust public education campaigns. By using these tactics, the nation saw a nearly 25 percent drop in the number of fatalities between 2005 and 2014, including a record low in 2011,” he said.

It was unclear what role, if any, seatbelt use and drunken driving had in the increase.

The big jump in 2015 deaths came in four states grouped by NHTSA — Montana, Idaho, Oregon and Washington — where fatalities increased by 20 percent. The next largest increase — 10 percent — was in six New England states. After that, one of the biggest regional increases — 9 percent — came in a grouping of Mid-Atlantic states to which NHTSA appended Kentucky.

The only decrease — 1 percent — in fatalities came in a group comprising Texas, New Mexico, Oklahoma, Mississippi and Louisiana.

There was no immediate explanation for the regional disparities.

One state that has released its 2015 data, Michigan, may be representative of the overall trend. Michigan State Police said traffic deaths were up 10 percent last year, but deaths involving teenagers, alcohol and motorcyclists were all up by more than 20 percent. Bicyclist fatalities were up 57 percent.

Adkins said he would await a more definitive breakdown of nationwide accident causes but suggested that a combination of factors may be in play.

“Typically, during periods of economic growth and low unemployment, we see the number of optional trips by motorists increasing,” he said. “Younger people may be employed now when they weren’t a few years ago, and they have a few bucks in their pocket so they can stop for a few beers on the way home from work or on the weekend.”