According to the NAFTA agreement of 1994, Mexican trucks were supposed to have full access to border states by 1995 and access to all of the USA by 2000 (Canadian trucks enjoy this same full access)!
The current "program" for letting Mexican trucks in is so protectionist that only 11 companies are going through the certification process. There are electronic monitors to measure how long the trucks are running. There are drug tests and English tests for the drivers. All things we do not mandate for drivers of American trucks. If it really is a safety issue, then these measures should be applied in a non-discriminatory manner.
The mix of protectionism and racism on display here is unseemly to say the least.