Data from the National Highway Traffic Safety Administration show more than 35,000 people were killed on the nation's roads and freeways last year, a 7.7 percent spike over 2014 and the most since 2008.
While final statistics showing the precise causes behind the rise in deaths won't be available until the fall, NHTSA notes that people drove 107.2 billion more miles in 2015, a 3.5-percent increase over 2014. The number of fatalities per 100 million miles driven rose to 1.12, from 1.08 in 2014.
In a press release, NHTSA administrator Mark Rosekind said an improving economy and falling gas prices, both of which contribute to increased driving, "explain only part of the increase" in road fatalities.
"Ninety-four percent of crashes can be tied back to a human choice or error," Rosekind said, "so we know we need to focus our efforts on improving human behavior while promoting vehicle technology that not only protects people in crashes, but helps prevent crashes in the first place.”
Among the largest increase in traffic deaths were: motorcyclists, up 9 percent; pedestrians, which increased 10 percent; and bicyclists, up 13 percent.
The northwestern U.S. saw the greatest increase in traffic deaths, with the toll rising 20 percent from 2014.
One analyst took the opposite view from Rosekind, suggesting that greater road congestion may be a culprit behind the rising death toll.
Writing at his "Antiplanner" website, Cato Institute senior fellow Randal O'Toole said: "There is some correlation between the changes in the amount of driving and fatality rates. This suggests that the changes in fatality rates are partly due to congestion that could have been avoided if cities and states had increased road capacities to keep up with traffic growth."
The Department of Transportation is pushing for more and better technology, rather than more tarmac, to reduce road fatalities. The department is promoting new "automated safety technologies" such as automatic, emergency braking systems on cars that it believes "could greatly decrease the number of crashes."
Earlier this year, the Transportation Department and automakers announced a deal under which automatic braking systems would be standard equipment on "99 percent of new vehicles" sold in the U.S. by 2022.
The department is also seeking to require new vehicle-to-vehicle communications systems on cars that can warn drivers about traffic hazards they might otherwise not be able to see, such as slow traffic over a hill, or blind spots at intersections.
Such technology, the department believes, "could help drivers avoid or mitigate 70 to 80 percent of vehicle crashes involving unimpaired drivers."
Impaired drivers remain a significant threat. The DOT says drunk driving is responsible for "close to one-third" of highway deaths, and its researchers are working on new technologies that won't allow a driver to start a car if monitoring systems detect elevated blood alcohol levels.
Transportation Secretary Anthony Foxx said in a statement that his department is "aggressively testing new safety technologies, new ways to improve driver behavior, and new ways to analyze the data we have," in order to reduce fatalities.
“Every American should be able to drive, ride or walk to their destination safely, every time,” Foxx said.