Highway safety experts were at a loss to explain why, but most speculated that rebounding economic confidence may have put more people on the roads.
"Traffic deaths drop in a recession, sometimes significantly," said Russ Rader, spokesman for the Insurance Institute for Highway Safety. "People who lose their jobs or are worried they may lose them don't take as many optional trips, like driving at night or on weekends, or going to parties or the bars. Once the economy improves, that driving comes back."
The number of roadway fatalities jumped by 13.4 percent in the first three months of this year, and the total for April, May and June was 5.3 percent higher than in 2011. For the six-month period, 16,290 people were killed, 1,340 higher than in the same time frame last year.
Although the recession has been seen as a factor, the decline in traffic fatalities began before the economic turmoil. After 43,510 people were killed in 2005, the number dropped each year until it reached 32,310 last year.
Increased use of seat belts, greater awareness of drunken driving, better highway design, installation of air bags and other auto safety improvements were given credit for the decline.
Barbara Harsha, executive director of the Governors Highway Safety Association, said the jump in fatalities was cause for concern, but she cautioned that half a year's data didn't constitute a trend.
"If there are increases over a three- or five-year period, then it's time to take a second look at what is being done and retool," Harsha said. "It's extremely difficult to maintain steady decreases over time, and the increase is not unexpected."
She shared Rader's assessment that people do more discretionary driving and get into more accidents as the economy improves.
"However, since [the total number of miles people are driving] went up only 1.2 percent, the economy isn't the whole answer to the puzzle," Harsha said. "The warm winter may also have contributed to the increase, because it meant a longer riding season for motorcyclists, more walking and biking."
Lynda Tran, spokesperson for the National Highway Traffic Safety Administration, which compiled the data, pointed out that the 9 percent increase in fatalities for the first half of this year is in comparison with 2011, when there were fewer traffic deaths than any year in the past six decades.
If the number of fatalities for all of 2012 were to increase by 9 percent, the total still would be more than 2,200 fewer than in 2008.
"If there is an increase, total fatalities will likely be less than they were ten years ago, possibly even five years ago," Harsha said. "That's because there have been effective programs for changing driver behavior, safer vehicles and improved roadways."
NHTSA collects traffic accident information from state law enforcement agencies. The data for the first six months of this year is preliminary and subject to revision by the time NHTSA's annual report on traffic fatalities for 2012 is released a year from now. The preliminary statistics did not include the state-by-state breakdown and analysis that will be contained in the final report.