Mexico is building seven border bridges and thousands of miles of highway to handle the boom in traffic from an expected free-trade agreement with the United States.

With help from private investors, President Carlos Salinas de Gortari set a plan in motion last year to improve Mexico's inadequate roads and customs system.Solidarity Bridge and border crossing at Colombia, now a cinder-block town of only 350 people, is the largest project.

A 66-mile highway linking Colombia to an improved road between Laredo, Texas, and Monterrey will cut the driving distance between the border and Monterrey to 140 miles from 180.

More than 2,480 miles of highways are to be built before the Mexican president's six-year term ends in 1994. Half will be financed through private investment.

The other bridges, under construction or planned, will be in Matamoros, Nuevo Laredo and Valle Hermosa, all in Tamaulipas state; Piedras Negras and Ciudad Acuna in Coahuila, and Ciudad Juarez in Chihuahua state.

Mexican officials say the improvements are needed to strengthen trade links with the United States and compete with the economic blocs of Europe and Asia.

Trucks crossing into Mexico often must wait for hours, then negotiate undivided, two-lane highways riddled with potholes.

"We don't need to be a rich country to have good roads, but we need good roads to become a rich country," said Carlos Joaquin Vallejo Rivero. He directs the state highway office in Nuevo Leon, the northeastern border state where the Colombia project is being built.

"It costs more and takes more time to transport things via bad roads because of higher insurance and loss of goods," he said.

Trade with the United States took off in 1986 when Mexico joined the General Agreement on Tariffs and Trade, lowering import duties on hundreds of foreign products and making exports of Mexican manufactured goods a priority.

From less than $30 billion in 1986, two-way trade rose to $52 billion in 1989. Mexico is the third-largest U.S. trading partner, after Canada and Japan.

A U.S.-Mexican trade agreement that would remove tariffs and other barriers is under discussion. It may be linked with the similar U.S.-Canadian pact of 1988 to create a single North American trade zone of 335 million people.

Salinas and President Bush are to discuss the proposal at a meeting Nov. 26-27 in Monterrey.

"All indications are that we are going to have even more intensive trade with the United States, and we are just not prepared," said Antonio Fuentes Flores, director of the Trust for the Development of Nuevo Leon.

Fuentes, whose organization is overseeing construction, said the 820-acre Solidarity site will include an eight-lane bridge, storage terminals, refrigerated warehouses and hotel.

It will more than triple the size of Colombia, a town 10 blocks square populated mostly by farmers.

Many people didn't think a new bridge was justified "because we already had a crossing at Laredo," Fuentes said. Laredo is the United States' biggest inland port of entry, with 1,600 trucks crossing daily.

"But we looked at projections of how much traffic would increase in the next few years," he said, and "concluded that we didn't need to just build a bridge, but we needed to build a new concept of border crossings."

Nuevo Leon was an ideal site, Fuentes said, because more than half the merchandise moving south through Laredo goes to Monterrey, the state capital and Mexico's leading industrial city.

Monterrey businessmen wanted a border crossing in Nuevo Leon so they would not have to send products through Nuevo Laredo in Tamaulipas state. They say customs officials and corrupt unions in Nuevo Laredo inflate the cost.

The Mexican side of the bridge and its support facilities is to cost $49 million, split between the federal government and private investors.

Laredo, Texas, is financing the U.S. side, at $62 million, with help from the state government and Washington. The site is Dolores, a town 18 miles from Laredo.

Four new highways in Nuevo Leon, totaling 350 miles, are being built with $584 million in private funds.