The study said people might have used their vehicles more frequently to avoid increasingly bad weather - rising temperatures and heavier rainfall - resulting in an increased number of deaths on the road.

The finding challenges a widespread notion that 2015's hike was the result of increased cell-phone usage by motorists.

Road deaths in the United States climbed 7.2 percent in 2015 over the previous year to 35,200, running counter to a five-decade trend of declining fatalities, according to the U.S. Transportation Department.

Traffic deaths climbed nearly 8 percent in the first nine months of 2016, government data showed.

Combining government data for the 100 most densely-populated U.S. counties for miles driven, vehicle fatalities and weather, researcher Leon Robertson found that motorists clock up extra miles as temperatures and precipitation rates rose.

When temperature rose by a degree Fahrenheit (0.5 Celsius), vehicles were driven an additional 60 miles (95 kms) per person over a year, Robertson said in the study, which was published in the academic journal Injury Prevention.

Using mathematical models, the retired Yale University epidemiologist also found that for every additional inch (2.5 cm) of rainfall, cars and trucks racked up an average of 66 more miles (105 kms) per motorist for a year.

Hotter than normal outdoors temperatures likely accounted for most of the extra deaths in 2015, Robertson said.

"If millions more people drive cars because the temperature is getting warmer ... then that adds up to a lot of miles," he told the Thomson Reuters Foundation.

"Mainly it's a simple multiplication."

In all, motorists in areas he studied may have racked up as many 13.6 billion more miles of travel, Roberston estimated.

He dismissed the idea that increased cell phones behind the wheel was to blame for the 2015 hike, a theory favoured by some safety officials.

Using a cell phone while driving has been associated with an increased risks of car crashes, but a national survey of observed cell phone use by drivers cited in the study found no change between 2014 and 2015.

In a statement, Roberston said the trend could continue as climate change intensifies.

"As temperatures continue to increase from heat-trapping gases in the atmosphere, road deaths will likely increase more than expected unless there are major mitigating countermeasures," he said in a statement.

The U.N. Intergovernmental Panel on Climate Change, which includes more than 1,300 scientists, forecasts a global temperature rise of as much as 7 to 8 degrees Fahrenheit (4-5 Celsius) by 2100, short of significant greenhouse gas emissions cuts. (Reporting by Sebastien Malo @sebastienmalo, Editing by Lyndsay Griffiths. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights, climate change and resilience. Visit http://news.trust.org)