If those big, snorting tractor-trailers tailing your car or blowing by on either side make you nervous, don't even think about how safe any of them may be. Many are fine, but too many are mammoth rattletraps. Ask the authorities who inspect them.

When Loudoun County Sheriff Stephen O. Simpson's traffic safety unit set up an inspection checkpoint on a recent Monday on Route 50 at South Riding Boulevard, he knew what to expect. For deputy sheriffs Vernon Foster and Chris Rizzo, spotting unsafe trucks is a snap: A good 35 to 50 percent of the commercial vehicles they pull over have safety violations; and during this five-hour operation, run in cooperation with the Virginia State Police and the Fairfax County Police Department, 10 of the 39 trucks inspected were taken out of service -- not allowed to drive away until repairs were made -- while 55 safety violations also were cited.

What you see on these inspections is not just dead headlights, faulty turn signals or bad wipers, common though they can be. One huge wheel -- about 200 pounds that could spin off at, say, 65 miles an hour -- has one lug stud seared off and another lug that can be loosened by hand. Another vehicle is way overweight with a huge load of sod, piled high and held inadequately by straps that are fraying and already shifting precariously to one side. Tractor-to-trailer connections cracking under strain, steering and brake problems, bald tires -- each of these vehicles is a major accident waiting to happen.

Why do drivers take off in vehicles this dangerous? Many fear that if they report safety problems they will be fired. Authorities say they are occasionally signaled by drivers who want to be pulled over and written up. At least Loudoun authorities are out patrolling regularly for unsafe trucks. Not so in every county in Virginia and certainly not in great stretches of the country. The vehicles themselves are only part of the safety problem: the condition of drivers is another area begging for more enforcement.

An average of 102 Americans die each week in accidents involving large trucks. "It's the equivalent of a major airplane crash every two weeks," says Virginia Rep. Frank Wolf, chairman of the House Appropriations subcommittee on transportation. If a major plane crash happened every other week, Mr. Wolf adds, Congress and the administration would be intensely focused on safety improvements. Transportation Secretary Rodney Slater has voiced concerns as well. In his home state of Arkansas, 135 people were killed in 1997 in crashes involving large trucks.

Walter B. McCormick Jr., president of the American Trucking Association, says the statistics are misleading, noting that the rate of fatal accidents per vehicle miles traveled, rather than the death tolls, is a more accurate measure of truck safety. Yet highway safety groups such as Public Citizen say that large trucks have more fatal crashes per mile traveled than passenger vehicles; and in fatal crashes involving a car and a truck, more than 95 percent of the fatalities are car occupants.

Rep. Wolf and a number of highway safety organizations are pressing legislation that would consolidate federal highway safety responsibilities under a single agency best organized to do the job. The measure would relocate the Transportation Department's Office of Motor Carriers -- which oversees trucking laws -- out of the Federal Highway Administration. It could go to the National Highway Traffic Safety Administration, which focuses on vehicle safety. In February, the Transportation Department's inspector general reported to Wolf's subcommittee that the Office of Motor Carriers had "shifted emphasis from enforcement to a more collaborative approach to safety." OMC inspectors were conducting fewer than two compliance reviews of trucking companies per month, with 12 percent of the violations resulting in penalties that had been bargained down to nearly half of the originally assessed amounts.

Merely shuffling an agency around won't in itself produce better federal oversight of truck safety. But given the history of the Office of Motor Carriers, truck safety ought to be the responsibility of an agency whose primary concern is safety, not the building and maintenance of roads.

The writer is a member of the editorial page staff.