A woman rides her bike in Berlin. Cyclists were among the most vulnerable to heat-related road accidents. Sean Gallup/Getty Images

A 7% spike in road deaths in 2015 has been attributed to unusually warm weather across the US.

The sudden increase in fatalities bucked a 35-year trend of decreasing road fatalities. It was previously attributed to the use of mobile phones, but this trend didn't seem to fit the sharp increase in deaths, according to a study in the journal Injury Prevention.

"Cell phones didn't change that much in one year. It was a record temperature year in the US, and people go out more when it's warmer," study author Leon Robertson, a retired epidemiologist at Yale University's School of Public Health, told IBTimes UK.

There was an average increase of 1.5C from 2014 to 2015. When the weather is warmer, people tend to take more trips.

"People consult the weather to decide whether to ride a bike or walk," Robertson said. "It's just a question of travel for any reason – recreation, visiting friends or going to play golf."

This potentially exposes them to greater risk of accident. Robertson found that in the 100 most densely populated counties in the US, people drove an extra 60 miles a year for every 1C increase in temperature. That added up to an additional 13.6 billion miles driven in 2015.

"More than half of the variation in deaths was explained by temperature," said Robertson.

Warmer weather hit the most vulnerable road users hardest – cyclist deaths went up 13%, pedestrian deaths went up 9.5% and motorcyclist deaths went up 8% from 2014 to 2015. Although a breakdown of how many people went walking or cycling by roads on given days is lacking, it's plausible people are more likely to get out on foot in warmer weather.

After accounting for factors such as precipitation, speed limits and drink-driving laws, the relationship with temperature was still significant.

Climate change has led to a slow average increase in temperatures on a global scale over decades. The spike in deaths was linked to a record high yearly temperature in just one year, following decades of decline due to tightening road safety laws. While this might seem counterintuitive, Robertson asserted that temperature strongly predicted a greater number of road deaths on a day-to-day basis.

As climate change progresses, record high temperature years - and other forms of extreme weather - are expected to become increasingly frequent. Analysis of 2016's data on road deaths, when it becomes available, will help to test the proposed link between temperature and fatalities. 2016 was the hottest year on record, and 2017 is predicted to come in a close second.

To counter this trend, Robertson recommended exercising in areas separated from cars and other traffic as much as possible.

"A lot of places are trying to create separate areas where there's no traffic, and some specific paths for bicycles and pedestrians. I would recommend people to continue to exercise in parks, fields, hiking trails – in places away from cars as much as possible."