The sheer size of semitrailers can make other drivers sharing the road nervous. Here's something else to consider the next time you're on the highway. A new report by the Government Accountability Office suggests that some 563,000 commercial license holders had qualified for full disability benefits, yet they continued to obtain drivers' licenses.

These disabilities can include vision, hearing or seizure problems. Not all serious medical conditions interfere with safe operation of a commercial vehicle, but some drivers identified in the report have conditions that should disqualify them from obtaining a license.

It is unclear how many of these licensed drivers actually are on the road, and only about 4 percent of all licensed drivers receive disability benefits. But it only takes one driver with a serious medical condition behind the wheel of a 70,000-pound semitrailer to cause injury or death.

According to an Associated Press review of 7.3 million commercial driver violations compiled by the Transportation Department in 2006, the latest year data were available, truckers in every state have been caught violating federal medical rules. Drivers were most frequently sanctioned in Texas, Maryland, Georgia, Florida, Indiana, Pennsylvania, Illinois, Michigan, Alabama, New Jersey, Minnesota and Ohio.

One Florida bus driver told federal investigators that he "occasionally blacks out and forgets things." The driver, who has lung disease, uses three daily inhalers to control his breathing. The man, who works as a substitute driver despite not having a medical certificate, has collected Social Security benefits since 1994.

In another case, four people were killed when a gasoline tanker plunged from an overpass and exploded in flames on Interstate 95 near Baltimore in January 2004. Maryland investigators concluded the driver had suffered a heart attack or other medical emergency, although his family disputed the findings.

The industry takes many steps to ensure the safety of its drivers, including drug testing. Truck drivers, for the most part, have impressive driving records.

It defies logic that trucking companies, concerned about the bottom line and insurance costs, would knowingly employ drivers who receive disability incomes for conditions that render them a hazard on the road. It defies understanding that a truck or bus driver, who has full appreciation of the risks posed when a driver is impaired by illness, medication or lack of sleep, would knowingly place other drivers and passengers in danger due to his or her condition.

The federal government and state licensing authorities need to more carefully scrutinize license applications and ferret out health-care providers who put the motoring public at risk by approving medically unfit people to drive. These are unacceptable risks.