Because of the United States' refusal to allow Mexican haulers onto its roads, about 4,500 Mexican trucking companies are suing the United States for $6 billion, according to Reuters. 

The United States had agreed under the North American Free Trade Agreement (NAFTA) with Mexico and Canada to let Mexican trucks use its highways. But with strong opposition from U.S. labor unions and consumer groups, the United States has confined the Mexican firms to a narrow border area. The labor unions and consumer groups have said Mexican truck safety standards are lax. 

The dispute has raised fears of a trade war. Mexico imposed tariffs on $2.4 billion worth of U.S. exports in March after the U.S. Congress ended a 2007 program that opened up American highways to some Mexican long-haul trucks. 

But Mexico plans no further sanctions and is open to a negotiated agreement, especially since total trade between the United States and Mexico was $368 billion last year.