WASHINGTON - Traffic fatalities rose 5 percent last year, according to an analysis of preliminary state data, reversing a seven-year decline in which the number of annual traffic deaths reached their lowest level in more than six decades.

An estimated 36,200 people died motor vehicle accidents in 2012, the National Safety Council said Tuesday. That's up from 34,600 deaths the previous year. It's the first increase since 2004 to 2005.

Crash injuries requiring medical care also rose 5 percent last year to 3.9 million, the council said. The estimates are based on monthly fatality data the council receives from every state and the District of Columbia.

The council and other safety advocates attributed the increase in part to more driving due to an improved economy and a mild winter last year.

While that may explain some of the increase, the rate of deaths also increased 4 percent to 1.23 deaths per 100 million vehicle miles traveled. The estimated annual population death rate was 11.49 deaths per 100,000 people, also an increase of 4 percent.

One explanation is that not only are people driving more as the economy improves, but they're also driving differently, said Michael Sivak, a professor at the University of Michigan Transportation Research Institute.

During the economic downturn, people tended to drive slower to conserve gas; there was less driving on rural roads, which are more risky than urban roads, and there were fewer freight shipments and thus fewer heavy trucks on the road, he said.

"Improvements in road safety that are based on economic factors are transient," Sivak said. "We should not be surprised that a part of the recent gain in road safety is beginning to disappear."

Increases in pedestrian and bicyclist deaths - a reflection of more people walking and biking in urban areas - may also be a factor, said Barbara Harsha, executive director of the Governors Highway Safety Association, which represents state highway safety offices.

Distracted driving is also suspected to be a factor, "but the distraction data is very, very difficult to get," she said.

Cars are safer than ever, with an array of new high-tech safety systems beginning to permeate the new car market. But many of the causes of highway deaths remain unchanged: Drivers not wearing seat belts, drunk driving, inexperienced teen drivers, unsafe trucks and motorcyclists not wearing helmets, said Jackie Gillan, president of Advocates for Highway and Auto Safety.

"I don't buy this theory that, 'Oh, we have safer cars, what else can we do?'" Gillan said. "There are many more things we can do. It's not rocket science. We know what the cure is."

The estimated cost of motor vehicle deaths, injuries and property damage in 2012 was $276.6 billion, the council said.

The costs include wage and productivity losses, medical expenses, administrative expenses, employer costs, and property damage.