Before 1982 Mexican truckers enjoyed access to U.S. highways. But lack of Mexican reciprocity led the U.S. to impose restrictions on that access.
These reciprocal access issues were supposed to have been resolved by NAFTA. In 1994 and 1995 the U.S. began to implement a phased extension of the existing restricted Mexican access. In the first phase, Mexican truckers would be allowed work in California, New Mexico, Arizona, and Texas. Later they would be allowed access to the rest of the U.S.
However, Clinton backed off in 1995 and suspended the reintroduction of Mexican truckers. This was either because of safety and environmental concerns or protectionist lobbying. The reason will depend on who you talk to. Efforts to resolve the situation continued and in September 2007 the U.S. Department of Transportation (DOT) was ready to implement a limited, one-year, demonstration project to allow Mexican truckers nation-wide access.