The Supreme Court ruled yesterday that the Bush administration can open U.S. roads to Mexican trucks as soon as it wants, overruling a lower court judgment that the government must first study the environmental effects.

The high court decision clears the way for the administration to take the final procedural steps needed to allow Mexican rigs to carry freight deep inside the United States. Labor and environmental activists who have fought to block the trucks from American highways said Congress might still erect more obstacles. But the administration praised the ruling as eliminating one of the last hurdles before the border is opened much more widely than it is now, in accord with the North American Free Trade Agreement.

"I welcome the Supreme Court's unanimous ruling," Transportation Secretary Norman Y. Mineta said in a statement, adding that once final safety procedures are in place, "our two nations will be able to finally follow through on an important international obligation in a way that maintains the Bush administration's strong commitment to safety."

The groups that lost the case contended that the court's decision would lead to increased pollution and road congestion, especially in states along the U.S.-Mexican border, where most of the Mexican truck traffic is expected to go. "This ruling gives a green light to allow trucks to cross the border with no regard for their effect on the environment," said Joan Claybrook, the president of Public Citizen, an advocacy group founded by Ralph Nader.

The long-running saga over trucks has been one of the most heated controversies surrounding NAFTA. Critics of the pact argue that it exposes Americans to Third World hazards, while free traders accuse Washington of caving in to special interests and reneging on its promises.

Under NAFTA, which went into effect in 1994, the United States was supposed to phase out restrictions on Mexican trucks crossing the border by 2000, provided those trucks meet U.S. safety standards. But under pressure from members of Congress and the Teamsters union, which feared losing jobs to low-wage Mexican drivers, the Clinton administration maintained the existing barriers, citing safety concerns. As a result, Mexican trucks have been confined to a 20-mile zone along the border, where they transfer their loads to U.S. carriers in cities such as San Diego and Laredo, Tex.

The Bush administration vowed to open the border in 2001 after a NAFTA panel held that Washington was violating the agreement. But, again under congressional pressure, the administration agreed to develop a set of safeguards, including placing inspectors at each border crossing. After the safeguards were finalized in November 2002, opponents filed a lawsuit arguing that the government had failed to meet the legal requirement for conducting an environmental study. The 9th Circuit Court of Appeals agreed; that was the decision that the Supreme Court overturned yesterday.

The ruling does not mean that Mexican trucks will start rumbling onto American highways immediately. The administration is still working with Mexican officials on issues of how it will conduct safety audits of trucking companies, including the question of whether U.S. inspectors will go into Mexico. Hundreds of applications from Mexican trucking firms must also be processed and approved.

"I would suspect you will see some Mexican trucks" on U.S. roads "before the end of the year," said Luis de la Calle, Mexico's former deputy trade minister who has been working on the issue for nine years.

Even then, Mexican trucks aren't expected to venture beyond the border states in large numbers, because they can carry freight only on routes between Mexico and the United States, not from one point inside the United States to another.

"In all likelihood, the Mexican companies that get operating authority will drive to places like Houston and Dallas -- the major load centers for vessels and trucks," said James R. Giermanski, a professor at Belmont Abbey College in Belmont, N.C., who has testified before Congress on the issue.

Some U.S. companies that have factories on the Mexican side of the border, such as Emerson Electric Co., might use Mexican trucks to ferry their goods farther north to their main plants in the United States, Giermanski said. But in general, "the Mexicans will want to go to a hub, where there's lots of cargo, because they can't afford to go back to Mexico empty," he said. "They're not going to drive to New York, because they would need to have international cargo to go back."

Correspondent Mary Jordan in Mexico City contributed to this report.

Commercial trucks carrying goods bound for the United States lined up in Nogales, Mexico, before entering the U.S., in this 2001 photo.