I asked this very question regarding weapons, and also about DUI's. Here is what I was told by a government employee. Say you are pulled over and have your firearm, and your MMJ, and you are completely white eyed and no issues, in the middle of a Sunday after attending church, and your firearm is secured in a legal fashion. First, if they find you with MMJ, even with your certified card, and you are operating a motor vehicle, you are popped with a DUI. They will follow the same path that you are impaired with a firearm as well and that is another charge. Here is the issue: The fact that MMJ stays in your system, even though you may not be impaired at that moment, you will test positive when they take your blood. Now there are many field phlebotomist that will pull up on scene, and extract your blood on the side of the road. Your driver's license gives them the right to take your blood.
I was told that if you carry MMJ, completely legal as stated on the books, you can never drive a car or have a firearm.