UEVO LAREDO, Mexico, Aug. 1 — About 1,500 miles from this border city, on Capitol Hill in Washington, the prevailing wisdom is that unsafe, badly maintained Mexican trucks are about to invade the United States unless Congress maintains a ban that has kept Mexican truckers from traveling more than a few miles inside the border.

But experts on both sides of the border say opponents of Mexican trucking, including the Teamsters union, have marshaled safety statistics that on close examination are misleading or incomplete.

There appear to be little or no data to prove that Mexican long- haul trucks and truckers are far more dangerous than their American counterparts, experts say.

The trucks that make the crossings along the 2,100-mile border with the United States are purposely older and less well-maintained vehicles used solely to travel the short distances from terminals on the Mexican side to depots on the American side. They would never be used for long hauls into the United States.

Those short-haul trucks are often forced to idle at clogged crossings and by law cannot travel more than 20 miles beyond the border. The American opponents of Mexican trucking have cited safety statistics on those trucks — 36 percent failed safety checks last year, according to the federal government — to block attempts to lift the 20-mile limit.

In approving a $60 billion transportation bill, the Senate voted today to continue limits on Mexican trucks and imposed new safety and insurance restrictions on Mexican trucking businesses. President Bush has threatened to veto the measure because it would block his plan for Mexican trucks.

People pushing to lift the ban on Mexican trucks said the fight would resume next month if the new rules were not changed in a compromise with the House. The White House, which wants the prohibitions lifted, said the Senate vote would hurt chances of improving ties to Mexico.

Mexican trucking owners dismiss the idea that they would send fleets of long-haul Mexican trucks roaming American roadways if Congress relented and the mileage limit and other rules that the Clinton administration unilaterally imposed on the North American Free Trade Agreement in 1995 are eliminated.

"That's not reality," said Roberto Quintanilla, president of CenSeCar, a large trucking company here in Nuevo Laredo, the gateway for 40 percent of the overland trade between Mexico and the United States.

"Neither Mexican nor American trucking companies have the wherewithal to make a buck on the other side of the border," Mr. Quintanilla said. "Someday, it might be good business to do it. But the politicians up there should come down here, spend a day on the border, see what's going on, learn a little about Nafta."

Manuel Gómez García, president of a Mexican trucking association, Canacar, said: "It's never going to make economic sense to send Mexican trucks to New York or Chicago. No way. It would be more practical for us to close up all our operations and reincorporate in the United States" — and that is a highly unlikely prospect.

Since Nafta took effect in 1994, fewer than 2 percent of the trucking companies in Mexico have applied to do business in the United States. Only the best-run best- financed and most professional could do so profitably, Mexican and American businessmen and officials said.

"This argument has nothing to do with safety," said Jim Giermanski, for many years an academic expert on cross-border commerce who is director of international business studies at Belmont Abbey College in Belmont, N.C. "It has everything to do with politics and money."

The image of Mexican truckers painted in Washington is overblown, with the threat posed by thousands of illiterate peasants barreling down Interstates, behind the wheels of 40-ton tractor- trailers with bad brakes.

"There is going to be blood on the highway," said Representative Peter A. DeFazio, Democrat of Oregon, one of hundreds of lawmakers who are trying to retain the mileage limit.

"Nafta is a trade pact," Representative David R. Obey, Democrat of Wisconsin, said during a House vote five weeks ago. "It is not a suicide pact."

The House voted to bar the Transportation Department from approving requests from Mexican truckers to operate in the United States.

In Mexico, as in the United States, there are two trucking businesses, long-haul companies that use newer, better-maintained vehicles, and short-haulers with more aged fleets who need to travel just short distances. "It's true that some of those trucks are in bad condition and that some of the operators don't have the best training," Mr. Gómez of the trucking group said. "But I know, and the Americans know better than to confuse the long-haul with the short-haul trucks."

Mexican products rolling into the United States arrive at a Mexican border depot on long-haul trucks. They are loaded onto short-haulers that go back and forth over the border between depots on each side. Finally, an American long-haul trucker takes the cargo from the American border depot to its United States destination. That makes at least three trucks to cross one line, although sometimes five are needed, experts said. The point of the Nafta provisions, Mexican and American officials said, was to streamline the process and cut costs.

"Everybody knows the movement of goods across the border today is immensely inefficient and so costly that it defeats Nafta's goals," said John Simpson, president of the Association of American Importers and Exporters and a former senior Treasury Department official.

In the Nafta debate in Washington, the foes of Mexican truckers relied on faulty Transportation Department figures that showed that 4.5 million Mexican trucks crossed into the United States in 1999. If a high percentage had safety problems, the opponents said, it stood to reason that great convoys of defective Mexican trucks would soon be crisscrossing the United States.

The Teamsters president, James P. Hoffa, said last month at a hearing that 36 percent of the Mexican trucks inspected on the American side of the border last year had something wrong with them, providing "a recipe for disaster."

Because American trucks inspected throughout the United States, Mr. Hoffa said, have a 25 percent failure rate, Mexican truckers threatened the safety of American families, not to mention the jobs of law-abiding Teamsters.

In fact, the correct number of trucks that crossed the border in 1999 was 63,000, each making many trips across the border, the Transportation Department now says.

They were short-haul trucks, making for an apples-and-oranges comparison with safety information. The one-quarter of American trucks that fail roadside inspections are overwhelmingly long-haul trucks, and not short-haul rigs, according to experts.

By comparison, short-haul trucks that transport cargo to and from rail yards in Kansas City, Mo., have a failure rate of 45 percent, according to the Kansas City police.

The American trucking industry has its own safety problems. Accidents that involve heavy trucks have killed an average of 5,000 people a year for 15 years. Similar figures from Mexico are not available.

The Transportation Department has told Congress that "once the border is open to long- haul traffic, the number and percentage of safety-compliant Mexican trucks will dramatically increase, because long-haul trucks will be different from, and in better condition, than the shorter- haul trucks."

Even if the rules change to let long-haul Mexican truckers enter the United States, Mexican trucking companies would have to apply to the Transportation Department and meet United States legal standards to operate in the United States.

Given the opportunity to apply, most Mexican trucking companies have declined. They would have to clear their practices with the Transportation Department, review their books with the Internal Revenue Service and pay federal, state, local and payroll taxes for the drivers.

"The fact is," Mr. Giermanski said, "if we open the border tomorrow, nothing will change. There's no way the Mexican long- haul trucker can drive willy-nilly around the United States."

Still, for Mario Galván, a Mexican short-haul trucker who was waiting in the sun at Nuevo Laredo to cross the World Trade Bridge into Texas, the prospect of more open borders and open roads sounded enticing.

"It's a good idea," Mr. Galván said. "I guess it all depends on whether the politicians in the United States see that we have the intelligence and cautiousness necessary to do the job."