The number of traffic-related fatalities in the United States rose sharply in the first nine months of the year, according to the Department of Transportation's National Highway Traffic Safety Administration.

In its latest estimate of deaths on U.S. roads, the NHTSA calculated that 26,000 people died in car accidents between January and September of 2015. This marked a 9.3 percent increase over 2014, when there were 23,976 traffic fatalities in the first nine months of the year.

Traffic fatalities in the United States have declined steadily in the past decade. A total of 32,141 people were killed in car accidents in 2005. The number of fatalities on the road declined 22 percent between 2000 and 2014.

However, the number of deaths from car accidents has gone up for four quarters in a row. This change started in the period between October and December of 2014, when 8,879 people died in traffic accidents. This number marked a 4.5 percent increase over the same period in 2013.

There were 7,375 fatalities in the first quarter of 2015, a year-over-year increase of 7.8 percent. Fatalities rose 8.3 percent in the second quarter to 8,850.

In the third quarter, the number of fatalities resulting from car accidents jumped 11.3 percent to 9,775. This was the largest increase since the first quarter of 2012, when traffic fatalities had an 11.8 percent year-over-year increase to 7,521.

"For decades, U.S. DOT has been driving safety improvements on our roads, and those efforts have resulted in a steady decline in highway deaths," said Secretary of Transportation Anthony Foxx. "But the apparent increase in 2015 is a signal that we need to do more."

The fatality rate also increased in the first three quarters of the year, going from 1.05 deaths per 100 million vehicle miles traveled in 2014 to 1.1 in 2015. The rate rose from 0.99 to 1.01 in the first quarter, 1.03 to 1.08 in the second quarter, and 1.11 to 1.19 in the third quarter. However, it was down from 1.46 for the year of 2005.

Each of the NHTSA's 10 geographical regions saw traffic fatalities go up in the first nine months of 2015, but the increase varied from place to place. Region 10—consisting of Idaho, Montana, Oregon, and Washington—had the largest increase at 20 percent. Fatalities were up 16 percent in Region 4, which includes Alabama, Florida, Georgia, South Carolina, and Tennessee. The New England states had the third largest increase at 14 percent.

The NHTSA said it is too early to determine what factors may be influencing the increase in fatalities. However, its report does note that the vehicle miles traveled in the first nine months of 2015 rose by 3.5 percent, or 80.2 billion miles.

In addition, the NHTSA says human factors contribute to 94 percent of vehicle crashes. The department recently launched a series of safety summits aimed at addressing behavioral problems such as speeding, failure to use seat belts, and the decision to drive while drunk, drugged, distracted, or drowsy. The NHTSA says it is also looking to develop ways to protect more vulnerable road users such as bicyclists and pedestrians.

"We're seeing red flags across the U.S., and we're not waiting for the situation to develop further," said NHTSA Administrator Mark Rosekind. "It's time to drive behavioral changes in traffic safety, and that means taking on new initiatives and addressing persistent issues like drunk driving and failure to wear seat belts."

The NHTSA uses data from several sources, including its own Fatality Analysis Reporting System, to estimate traffic deaths for each quarter. FARS tabulates accidents involving a vehicle traveling on a public roadway where at least one person dies within 30 days.

The estimates are typically revised as the NHTSA updates its information. The department expects to release information on the total fatalities for 2015 this spring and finalize its 2014 data in the autumn.