U.S. traffic deaths rose 9 percent in the first half of 2012 compared with the same period last year, breaking a 5-year downward trend, according to preliminary data that experts cannot yet explain.
Road accidents killed 16,290 people from January through June, the most since 2009, the U.S. National Highway Traffic Safety Administration said in a preliminary report that does not examine causes.
A final report of vetted and analyzed data could take a year or more.
In the meantime experts can only guess about the causes, which could be linked to the weather, the economy, gasoline prices or traffic safety scourges such as the increase in texting or the use of synthetic or prescription drugs.
"While it's too soon to speculate on the contributing factors of any increase in deaths on our roadways, NHTSA is closely monitoring the data," administration spokeswoman Lynda Tran said in a statement.
After what the administration called a "historic" downward trend that resulted in a 60-year low for traffic deaths in 2011, "We may just be going back to the way it was before," said Barbara Harsha, executive director of the Governors Highway Safety Association.
Harsha's group advocates bans on texting while driving and the use of any hand-held cellphone. It also favors making it mandatory for convicted drunk drivers to use an ignition interlock device, which forces the driver to blow into an attachment that prevents the car from starting if too much alcohol is detected. Seventeen states require them.
"Clearly there's room for improvement in distracted driving - we would like all 50 states to pass texting bans," Harsha said. "We'd practically eliminate alcohol impaired driving if we could get all offenders to use an ignition interlock." (Editing by Barbara Goldberg and Mohammad Zargham)