Because THC lingers in the fatty tissue, drug tests for it can't can't pinpoint when the drug was ingested. With a tightening labor market, many Oregon employers are no longer including THC in their drug screening protocol. I imagine that within the next 20 years most states will legalize both medical and recreational marijuana, as big corporations enter the market and wield their influence over lawmakers. This situation will require better testing. Was Jim Bob stoned when he tumped the backhoe, or was that THC just a result of his previous night's pain relief? How much THC in the blood makes you legally under the influence? Is it the same for everyone? What happens when someone goes on a crash diet and metabolizes 10 pounds of THC saturated fat? Lots of questions to be addressed, probably in the courts.
Personally, I only smoke before bedtime so my back doesn't keep me awake all night. There is no way I would ever consider operating any complicated or dangerous equipment in that condition. I once hunted with some stoners. Once was enough.