WASHINGTON -- Hundreds of thousands of tractor-trailer and bus drivers in the United States carry commercial driver's licenses despite also qualifying for full federal disability payments, and some of those drivers have suffered seizures, heart attacks or unconscious spells, according to a new U.S. safety study obtained by The Associated Press.

The problems threatening highway travelers persist despite years of government warnings and hundreds of deaths and injuries blamed on commercial truck and bus drivers who blacked out, collapsed or suffered major health problems behind the wheels of vehicles that can weigh 40 tons or more.

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.