Its the law of unintended consequences. And like it or not the doc has liability.
Kinda like you saying to the cop, I dont know what the speed limit is, there have been no signs since I got on this street. You think that is going to relieve you of the responsibility of knowing what the speed limit is?
The doc does have a conflict of interest in this case, much like a lawyer, and should have refered the driver to another doc. Or at the very least, should not have prescribed something to knock the driver out of his/her job.
My personal doc knows what the DOT rules are, and there are many times he would have prescribed something else, but it would have interfered with the safe opperation of the delivery vehicle.
Normally, I dont like the idea of suing, but the doc has responsibilities when it comes to your income loss during the time off. If it is a longer time than you can afford, see a lawyer.