Crawling along the hot concrete in the rectangular shade of an 18-wheeler's trailer, a federal inspector searched for leaky brake lines, loose lug nuts and bald tires.

This type of scrutiny, conducted Tuesday at El Paso's Bridge of the Americas, will keep roads safe, even after Mexican trucks are given access to the nation's highways, U.S. Department of Transportation (search) officials said.

Federal officials scheduled the demonstration after Monday's Supreme Court ruling clearing the way for implementation of a North American Free Trade Agreement (search) provision allowing Mexican trucks (searchinto the United States.

The Bush administration has supported opening the nation's Southwest border to Mexican commercial vehicles.

Brian Turmail, a Transportation Department spokesman in Washington, D.C., said he couldn't predict when the borders will open. He said a safety audit program for Mexican trucking companies has to be worked out first.

Unions, environmentalists and consumer advocates say it won't be easy to hold those trucks to the same standards applied to U.S. trucks, which could mean more pollution and accidents on the nation's highways.

"There's no possible way" they can inspect all the Mexican trucks, said Darryl McKenzie, a business agent for the Teamsters Local 745 in El Paso.

McKenzie said businesses springing up on the Mexican side of the border to take advantage of cheaper labor will force some U.S. carriers to push their employees to drive longer hours and to stretch out maintenance intervals.

In anticipation of the change, more inspectors have been hired and the number of inspections has increased in recent years. At the same time, Mexican trucking companies have improved drastically, said Elida Cruz, a Federal Motor Carriers Safety Administration border safety inspector.

"They've improved because we have impacted them so much by checking them constantly," she said.

In 2002, the most recent year for which statistics are available, there were more than 4.7 million crossings into the United States on the nation's Southwest border.

Since 1999, inspections have steadily increased and the percentage of Mexican vehicles taken off the road because of safety problems has steadily declined, according to the statistics from the Transportation Department's safety administration.

Every truck that will travel beyond a commercial buffer zone must pass the most rigorous safety administration inspection, said Ruben Torres, an El Paso border auditor supervisor for the safety administration. The buffer zone ends 20 miles or less from the border, where most Mexican truck companies pass their loads to U.S. haulers.

Under the "Level 1" inspection, the federal inspector checks the driver's license, insurance and crossing papers. Then he works his way counterclockwise around the truck. He checks steering, brakes, lights, tires and more.

Trucks that don't meet all the requirements can be towed to a repair facility or back to the border - at the hauler's expense.

Torres said trucks that stay within the buffer zone will be subjected at least to a walk-around check. It is up to the inspector whether a more in-depth inspection is necessary, he said.