The report found medical problems in some truckers that make it dangerous for them to operate big rigs.

Medical problems led to a crash that killed four people two years ago in Callaway County. Just last week, a jury acquitted the driver of that truck on four counts of second-degree involuntary manslaugter. The jury handed down that ruling after his lawyers argued in court that a diabetic episode put him in "an altered state of conciousness."

One of the victims described the scene of the accident as a war zone and the new study shows that horrible scene could be common, with more than 500,000 drivers with health problems behind the wheel. Some of the drivers have suffered seizures, heart attacks, or unconscious spells. Truck driver Don Coffman says more should be done to prevent this problem.

"The people with medical problems are slipping through, you know getting in the cracks and all it does is make a bad name for everyone else out there whose driving legally," Coffman said.

Coffman says truck companies should get tougher on guidelines.

"I think it's up to the company now to do this," Coffman said. "The company's the one that's going to be at fault when this truck driver hits a family."

Zoe Smith, a victim of the 2006  accident says she's frightened of the GAO study results and more investigations should be done. She's not the only one worried about unfit truckers.

"Maybe there should be more tests or have certain different people not be able to drive or be truckdrivers because of their health reasons," driver Jennifer Chamberlain said.

The report also says problems have threatened highway travelers for years despite years of government warnings and hundreds of deaths and injuries blamed on commercial truck drivers with medical incidents while driving.

The U.S. agency responsible for cracking down on unfit truckers, the Federal Motor Carrier Safety Administration, acknowledges it hasn't completed any of eight recommendations that U.S. safety regulators have proposed since 2001.