Don’t forget that in 2015 the NAFTA requirements that allow Mexican trucks on US roads were fully put into effect so now we’ve got heavily polluting Mexican trucks on Southern California roads now.
Is there a quota on that ? If not, I predict this will follow the same pattern it took in Western Europe, where low-cost of labor Eastern Europe companies are slowly taking over long distance trucking. The limiter of the rate of change is the number of available EE long-distance drivers, which the companies hiring have vacuumed clean.