The number of bicyclists killed in motor vehicle crashes on U.S. roads climbed 16 percent between 2010 and 2012, a new study shows.

The vast majority of cyclists who died were adult men, most were riding without helmets, and more than a quarter were legally drunk, the research found.

Allan Williams, a Maryland highway safety consultant who wrote the report for the Governors Highway Safety Association, attributed the increase in bicycle fatalities to more adults cycling, particularly in urban areas.

“The reason is there’s simply more biking,” Williams told Reuters Health. “It’s an urban phenomenon involving mostly adult males, and it’s a huge change from when mostly children were killed as bicyclists.”

“Bicycling is widely encouraged for health and environmental benefits. This study suggests we need to pay more attention to protect bicyclists when they’re out there on the road with motor vehicles,” he said.

Williams examined bicycle deaths from 2010 to 2012 as reported to the Insurance Institute for Highway Safety’s Fatality Analysis Reporting System. Bicyclists killed in motor vehicle crashes rose from 621 in 2010 to 722 in 2012, his study found.

While bike deaths shot up 16 percent, vehicle fatalities increased just 1 percent, the report says.

Despite the increase, the number of people dying in bike accidents remains among the lowest since 1975, when 1,003 cyclists died in the first year the data was compiled.

Since then, the demographic profile of bike crash victims has changed. In 1975, only 21 percent of bicyclists who died were adults, compared to 84 percent in 2012, the study found.

The researchers also found that nearly three-quarters of bicyclists killed in 2012 were adult males.

Most of the deaths were in urban areas, and 54 percent occurred in six states – California, Florida, Illinois, New York, Michigan and Texas. The proportion of deaths in urban areas rose from 50 percent in 1975 to 69 percent in 2012, the report says.

Failure to wear helmets and alcohol impairment were contributing factors in the bicyclist deaths. While 21 states and the District of Columbia require children to strap on headgear, no state mandates helmets for adult bicyclists. Prior research has shown that Australian and Canadian laws mandating helmets for adults led to increased use and a reduction in injuries, the report says.

In 2012, only 17 percent of bicyclists killed were wearing helmets, the study says.

At the same time, 28 percent of cyclists killed who were 16 and older had blood-alcohol levels above the legal limit, the study found.

Williams said the number of intoxicated bicyclists surprised him.

“You think of bicycling as a healthy physical activity, but there’s a subset that have gone astray from that,” he said.

Transportation engineer Offer Grembek called the study results “alarming” but said the data were too limited to draw conclusions from it.

Grembek is from the University of California, Berkeley, Safe Transportation Research and Education Center and was not involved in the current study.

Unlike with car fatalities, police do not collect information about bicycle-related infrastructure, such as distance from home or whether the accident occurred in an area with a bike lane, Grembek told Reuters Health.

There also is no definitive data about the number of bicyclists using the roads, he said.

“As researchers, we don’t have hard data to say is this increase in fatalities more or less than what we expected,” he said.

“The increase in bicycle fatalities is alarming, and we need more data and resources to explain why it’s happening and more importantly to understand how it can be reduced,” he said.

But Williams said he does understand the increase.

“We know what’s going on out there,” he said. “There’s a lot more commuting by adults.”