By Joan Lowy, Associated Press

WASHINGTON (AP) — The number of traffic deaths in the United States rose 8 percent from 2014 to 2015, the largest year-to-year percentage increase in a half-century, according to preliminary estimates Wednesday by the National Safety Council.

About 38,300 people were killed on U.S. roads, and 4.4 million people were seriously injured, the council said. That would make 2015 the deadliest driving year since 2008.

The council said a stronger economy and lower unemployment rates were probably among the key factors, along with lower gas prices. With driving more affordable, more people are on the road. Average gas prices were 28 percent lower last year than in 2014 and are projected to continue dropping this year.

The Transportation Department estimates a 3.5 percent increase in the number of miles driven by motorists in the United States in 2015.

The council's estimates correspond with one from the National Highway Traffic Safety Administration, which also found an 8 percent increase in fatalities for the first half of 2015.

Among the biggest increases last year were in Oregon (27 percent), Georgia (22 percent), Florida (18 percent) and South Carolina (16 percent). Only 13 states showed improvement, including New Mexico (a 20 percent drop), Kansas (down 7 percent) and New Jersey (lower by 2 percent).

The estimated cost of motor-vehicle deaths, injuries and property damage in 2015 was $412.1 billion, according to the council, a nonprofit organization created by Congress to promote safety. That total includes wage and productivity losses, medical expenses, administrative expenses, employer costs and property damage.

"These numbers are serving notice: Americans take their safety on the roadways for granted," said Deborah A.P. Hersman, the council's president and CEO.

To Learn More:

Deadliest State for Driving—Montana; Least Deadly—Massachusetts (by Noel Brinkerhoff and Steve Straehley, AllGov)

                Poor People More Likely to Die in Auto Accidents (by Noel Brinkerhoff, AllGov)

                Traffic Fatality Rate Hits Historic Low (by Noel Brinkerhoff, AllGov)

Drivers in Republican-Leaning States more likely to Die in Accidents than those in Democratic States (by Noel Brinkerhoff, AllGov)