Various factors indicate that phone-related road deaths are underestimated, experts say.

(HealthDay)—The number of fatal crashes involving drivers distracted by cellphones is vastly under-reported in the United States, according to a new study.

National Safety Council researchers looked at 180 fatal crashes nationwide from 2009 to 2011 where evidence indicated driver use. Of the crashes in 2011, only 52 percent were labeled as cellphone-related in federal .

"We believe the number of crashes involving cellphone use is much greater than what is being reported," Janet Froetscher, president and CEO of the National Safety Council, said in a council news release. "Many factors—from drivers not admitting cellphone use, to a lack of consistency in crash reports being used to collect data at the scene—make it very challenging to determine an accurate number."

Even in cases where drivers admitted cellphone use before a fatal crash, only about half were entered as cellphone-related in the U.S. 's Fatal Analysis Reporting System.

The study also noted that there are an unknown number of cases in which the use of cellphones in fatal crashes is impossible to determine. An example would be a driver reading an email or text message who dies in a crash without any witnesses.

There were large differences in cellphone-related fatal crashes reported by states. For example, Tennessee reported 93 fatal crashes involving cellphone use in 2011 while New York reported only one, despite its much larger population. In the same year, Texas reported 40 such cases, while neighboring Louisiana reported none.

In 2012, U.S. increased for the first time in seven years. Based on available data, the National Safety Council estimates that 25 percent of all crashes involved cellphone use.

"The public should be aware that cellphone-involved are not accurately being reported," Bill Windsor, associate vice president of at Nationwide Mutual Insurance Company, said in the news release. The company partly funded the study.

"These statistics influence national prevention priorities, funding decisions, media attention, legislation and policy, even vehicle and roadway engineering. There are wide-ranging, negative ramifications to safety if a fatal crash factor is substantially under-reported, as appears to be the case of cellphone use in crashes," Windsor said.

More information: The U.S. National Highway Traffic Safety Administration has more about distracted driving.