This HND piece delves into a brief history of how the trucking industry became regulated in 1935, which messed things up so badly that much of it became de-regulated in 1980. Of course, new regs would follow shortly.
This gets us into the bizarro world of required physical exams for truck drivers. Let's be kind and say that the integrity of these exams varies quite a bit. At the bottom are "doctors" who hang out a truck stops, and will basically pass anyone, to legit MDs who...yes...occasionally have to fail drivers, which unfortunately means that they cannot work as drivers anymore.
The other problem is the inconsistency of what drugs they test for, and what drugs are prohibited, but are not tested for.
Read the complete article.