But among the small fraction of trucks that are inspected, authorities have found multiple loads of contraband, including nearly 13 tons of marijuana seized in a three-week period last spring.
Some experts now question whether the program makes sense in an environment where drug traffickers are willing to do almost anything to smuggle their shipments into the U.S.
The trusted-shipper system "just tells the bad guys who to target," said Dave McIntyre, former director of the Integrative Center for Homeland Security at Texas A&M University.
The program works like this: Participating companies agree to adopt certain security measures in exchange for fast entry into the U.S. They are required to put their employees through background checks, fence in their facilities and track their trucks. They also must work with subcontractors who also have been certified under the program, which is run by the U.S. Customs and Border Protection agency.
The government keeps the list of participants secret, citing national security and trade secrets. But some of the 9,500 companies who are part of the system advertise their membership to drum up business, making them targets for smugglers, who can then threaten drivers or offer them bribes.
More than half of all U.S. imports now come from companies in the program, called the Customs-Trade Partnership Against Terrorism, or C-TPAT. Mexican trucking companies make up only 6 percent of global membership in the system, but they account for half of its 71 security violations during the past two years.
Mexican trucking companies face higher scrutiny than others. They get a full customs inspection every year, instead of every three years like other participating companies.
The most common contraband is marijuana, officials say.
In March, a driver from Tijuana, Mexico, offered inspectors at the U.S. border paperwork showing his truck was filled with toilet paper. But a drug-sniffing dog alerted authorities to five tons of marijuana in a hidden compartment.
A week later, customs officers found three tons of marijuana in trucks carrying auto parts and racks. Five days after that, agents in El Paso, Texas, found more than four tons of marijuana in a tractor-trailer hauling another load of auto parts.
Stephen Flynn, senior fellow for Counterterrorism and National Security Studies at the Council on Foreign Relations, said truckers do not feel safe rejecting bribes, no matter what agreements their companies have made with the U.S. government.
"The basic vulnerability for a truck driver remains the 'plata-or-plomo' dilemma," Flynn said, using Spanish shorthand for taking a bribe or a bullet.
John Chaffin, a trade lawyer near San Diego, said he had worked with one Mexican trucking company that wanted to join the program, but then pulled out. He suspects participating companies feel pressure from drug gangs to help them smuggle drugs into the United States.
"Some Mexican truckers have figured out, 'I don't want someone thinking I'm a better target than someone else,"' Chaffin said.
Mexican authorities suspect a man who owned a participating trucking company in Aguascalientes, Mexico, was killed by drug gangs in July 2008. The slaying of Gerardo Medrano Ibarra is unsolved.
In Laredo, the border's busiest crossing, nearly 700 trucks a day pass through the lane at the World Trade Bridge reserved for trucks that are certified by the trusted-carrier program, each one pausing only for a matter of seconds.
Trucking companies have to electronically submit a list of each vehicle's cargo to customs officials at least 30 minutes before arriving at the bridge. Customs agents review them for risk factors that could trigger an inspection. Customs will not reveal those factors, but people familiar with the program say potential risks are judged based on the factory that is sending the goods, its location, the truck's route and other matters.
Required cable locks on the trailer doors are also checked, but smugglers have been known to cut them and carefully glue them back together or take the trailer doors off at the hinges without disturbing the locks.
Mexican trucking company owner Leonardo Varela Resendez joined the program because he did not want to lose clients.
At first glance, Autotransportes Varela Davila, a family trucking business with 54 tractor-trailers in Reynosa, Mexico, seemed the sort of low-tech operation smugglers would target. Then Varela pointed out the security cameras surrounding the yard, the guard at the front gate who took down a visitor's license plate number and the woman who tracks his trucks' whereabouts by computer.
"I have learned good things from the U.S. like this, and we understand it benefits companies and the U.S. too," Varela said. Nearby Varela is building a new yard for his trucks. It is larger, modern and will include 128 mounted security cameras, as well as an infirmary for giving drug tests to drivers.
Varela, the local delegate for the national trucking advocacy group, said he does not fear being targeted by drug smugglers.
"They dedicate themselves to their thing, and we do ours," he said.
Daniel B. Hastings Jr., owner of a customs house with offices at five ports of entry on the Texas-Mexico border, thinks the customs program works. He cited cases where a Mexican trucking company tracking a truck noticed an unscheduled stop en route to the bridge and phoned to alert U.S. customs.
"I think they're doing as good as they can with what they have to work with," he said.