Filming teens while they drive and blocking cell phone signals inside their cars may both help reduce distractions that lead to crashes, a small study suggests.

"We found a large, significant reduction in high-risk driving events like hard braking and sudden swerving," said lead study author Dr. Beth Ebel, director of the Harborview Injury Prevention and Research Center at the University of Washington School of Medicine in Seattle.

The number of teen drivers involved in fatal crashes has dropped by more than half over the past decade as safer vehicles hit the road and more young people received restricted licenses, according to a recent U.S. report.

But crashes remain a leading cause of preventable death for U.S. teens, said Ebel, who presented the study findings today at the Pediatric Academic Societies annual meeting in San Diego.

For six months, she and her colleagues followed 29 drivers, ages 15 to 18. Some drove with in-vehicle cameras, some were recorded and also had cell phone signals blocked inside the car, and a third group had no intervention.

The cameras started recording if teens slammed on the brakes or swerved, and the teens would see a light go on to indicate they were being filmed, Ebel said. Video clips could be emailed to parents.

Some teens also had cell phone signal blocking programs downloaded on their smartphones that block the ability to call or text when the car is running, Ebel said.

These interventions reduced high-risk driving events by 80 percent, compared to the teens who didn't have cameras or cell phone signal blockers. Teens drove more safely when calls and texts were blocked than with just the cameras, but the difference wasn't statistically significant.

In a separate study presented yesterday, Ebel and co-author Laura Blanar, also of Harborview, explored the crash risks associated with distracted driving among more than 100,000 Washington state drivers ages 16 to 18 in 2012.

A review of police citations and accident records showed that among teens cited for distracted driving, 31 percent were later involved in a crash, compared with 4 percent of drivers without a citation. Crashes were most likely if teens had been cited for inattention, followed by texting or taking on a cell phone. Risks were greater for younger drivers.

While the study may not account for multiple things going on in a car that can also contribute to crashes, "it's certainly evidence that people who get citations have a higher risk of future crashes," said David Kidd, a senior research scientist at the Insurance Institute for Highway Safety in Arlington, Virginia.

There also isn't a lot of research yet that proves the effectiveness of cell phone blocking, said Kidd, who wasn't involved in the study. But to the extent that cameras or cell phone blocking help parents monitor what teens do in the car, it can be helpful at reducing risky behavior, he said.

Cameras aren't a panacea, however.

A recent analysis by the AAA Foundation of 1,700 videos of teen drivers found that they didn't seem to alter their behavior even though they knew they were being filmed. In many crashes where teens were using cell phones, they failed to brake or steer away before impact, said Jennifer Ryan, an expert in teen driver safety at AAA.

"The camera gives the parent an opportunity to see what's happening in the car and to have that discussion with their teen about how to make better choices," said Ryan, who wasn't involved in the current study. "Nothing can replace a parent being there in the car, but in reality parents aren't always going to be there."