IMHO, you would think that the automotive industry, smelters, coal - all industries that pollute the air/land/water would have been pro-active.
They chose, obviously or profits sake, no to modernize and make incremental changes. They kept on doing business as usual. Let the gov't worry about cleaning up pollution. Well, guess what, gov't is doing what needs to be done. Truck owners have been aware of these necessary changes for years.
Like that old ad about oil changes "pay me now or pay me later".
The air where I live is somewhat cleaner that before, but with many more vehicles on the road, it's a never ending struggle. I have noticed a large drop is "gross polluter" looking vehicles.
As far as trucking companies avoiding California - I just don't see a drop is big rigs on the road. CA's economy, though under severe pressures, is too large to ignore!
I'll get off my box now.