By David Shepardson

WASHINGTON (Reuters) - U.S. traffic deaths rose 7.7 percent in 2015 over the previous year to 35,200, the highest number of people killed on U.S. roads since 2008, the government's preliminary estimate reviewed by Reuters shows.

The report was compiled by the U.S. National Highway Traffic Safety Administration (NHTSA) and submitted to Congress. It shows the percentage increase in traffic deaths in 2015 was the highest annual jump since 1966. U.S. officials cite an increase in vehicle miles traveled, lower gas prices and an improving economy as part of the reason for the increase.

The rise in traffic deaths was significantly higher than the overall 3.5 percent increase in U.S. vehicle miles traveled in 2015 to a record high 3.15 trillion miles.

The fatality rate rose to 1.12 deaths per 100 million miles traveled in 2015 - the highest rate since 2010.

The report said deaths among bicyclists rose 13 percent, pedestrians 10 percent and motorcyclists 9 percent. Fatal crashes involving young drivers were up 10 percent.

Jonathan Adkins, executive director of the Governors Highway Safety Association, called the increase "alarming."

"The good news is that the solutions to reducing traffic deaths aren't a mystery. They include strong laws coupled with highly-visible law enforcement and robust public education campaigns," Adkins said. His group represents state highway offices that implement safety programs on roads including prevention of speeding and alcohol impaired driving.

The increase in deaths in 2015 - 2,525 more deaths than in 2014 - is one reason why U.S. officials are working to provide guidance this summer to states and automakers on the deployment of autonomous vehicles.

NHTSA says the technology could dramatically decrease traffic deaths - in part because 94 percent of all road deaths are attributed to driver error.

News of the increased road deaths came as NHTSA is investigating the fatal crash of a driver in Florida who was using Tesla Motors Inc's Autopilot mode in its Model S sedan. Tesla said the crash was the first fatal crash in the more than 130 million miles that the semi-autonomous driving system has been used.

"As the economy has improved and gas prices have fallen, more Americans are driving more miles," NHTSA Administrator Dr. Mark Rosekind said in a statement. "We know we need to focus our efforts on improving human behavior while promoting vehicle technology that not only protects people in crashes but helps prevent crashes in the first place."

Driving is far safer now in the United States than it was in the past. In 1966, the fatality rate - measured as deaths per miles driven - was five times higher than today. In that year, nearly 51,000 people were killed on U.S. roads.

(Reporting by David Shepardson; Editing by Bill Trott and Frances Kerry)