Hersman said truck and bus inspections show rampant violation of federal safety rules.

Approximately 20 percent of trucks and buses that are inspected are pulled out of service for mechanical issues, and 7 percent of drivers are pulled off the road because of hours-of-service, record-keeping or other issues, Hersman said.

"In the end we see companies that are not put out of service until after they have a fatal accident," she said.

The motor carrier safety agency prohibited the Canada-based bus company, Mi Joo Tour & Travel, from operating in the United States following the Oregon accident.

The agency, part of the Transportation Department, defended its practices. It said federal and state inspectors conduct more than 3.5 million truck and bus inspections across the United States each year.

"In any instance where the driver or the vehicle or both are found to be in serious violation of any federal safety regulation and thus pose an imminent hazard to public safety, the driver, the vehicle or both are immediately placed out-of-service by the inspector," the agency said in a statement.

"Even if a violation is not severe enough to place a company out of service immediately, companies are required to correct all violations for which they are cited, and minor violations can result in increased frequency of roadside inspections by state and local law enforcement," the statement added.