Travelers can step onto a commercial aircraft with confidence that it has undergone regular inspections because federal law and accompanying regulations require them and the Federal Aviation Administration tracks the work.

But step onto a commercial bus operated by one of more than 4,000 carriers and, well, who knows? Even though nearly as many people travel by bus as by airplane - 720 million a year by plane, 700 million by bus - there is no consistent, direct federal oversight for bus inspections.

State governments are responsible for enforcing federal safety laws covering interstate buses, even though those vehicles regularly cross state lines. The states also are responsible for commercial buses that travel only within states.

There is a wide array of policies among state governments. Only 23 and the District of Columbia require annual inspections, for example, and some states use most of their safety enforcement budgets to target heavy trucks rather than buses. And many states have different rules for privately owned buses.

Following a series of crashes involving buses operated by discount carriers, the Federal Motor Carrier Safety Administration conducted "Operation Quick Strike" in 2013, which put 52 companies out of business and removed 340 unsafe buses from service.

But bus safety enforcement, just like aircraft safety, should be pro-active rather than reactive.

Congress in 2012 ordered a study by the Department of Transportation on whether it should mandate uniform bus safety regulation by the states. The report is due in 2015. If the study confirms that bus safety regulation is spotty, Congress should use the same hammer it has used for uniform state speed limits and drunk-driving thresholds. It should withhold federal highway funds from any state that does not comply with uniform federal safety standards for all buses.