Everywhere I have ever worked drug tests for pre-employment screening and also following any injury or accident. Problem is, marijuana just stays in your system for so darn long that you may have used it weeks before a work-related incident and still show up positive, and it's hard to determine for sure just how long it has been since the last use. So if you get in a crash that hurts someone and show up positive on that drug test, it doesn't matter that that positive result is from using yesterday, last week or even last month, you still look really really bad.
I believe that explains why many companies that require employees to drive still have a zero tolerance policy on marijuana, even if it is for medical use.