I have talked to both my insurance agent and lawyer about this extensively. Mainly because my insurance carrier requires anybody who cause an auto accident to get tested, any workmanship comp claims get tested, also if anyone causes more than 2k in property damage and I file a gl claim, they get tested. What was explained to me, is that it is treated the same as a person who is on a narcotic prescription such as Percocet. The problem with both is that it is easy to recognize if the person is high off their ***, but if they have just enough in their system to be drowsy or loose focus! they may not show any signs (such as their eyes). The problem with both of these is that the person could use the drug while on vacation and not working, come back 2 weeks later, get in an accident and still pop positive. The test doesn't show if it is just residual in their system or if they were still slightly under the influence.
Because of this, we have a zero tolerance policy, if their is any chance you could pop positive for any. Prescription which states not to drive/use equipment,then you do not do so if there is a chance you could pop positive.
Why do people not respect us as they do other tradesmen? Because every Tom, Dick, and Harry doesn't think he can be a plumber or electrician!