asking a driver in Spanish if he understood U.S. highway signs.
The truth is that the U.S. Department of Transportation had no way of knowing whether every truck was "checked" every time it crossed the border. According to a status report by DOT's inspector general, highway officials "did not have assurance that every Mexican truck and driver participating in the project had been checked when they crossed the border into the United States."
It's also worth remembering that the U.S.-Mexican border has been chronically understaffed. The Center for American Progress reports that "there also is widespread consensus that current staffing is far from enough when it comes to protecting the border."
Given the rampant violence in Mexico and drug smuggling into the U.S., you would think the government would want to inspect every truck every time it came into this country.