Is it legal to drive after using marijuana?

In Montana, it is illegal to drive on public roadways if your delta-9-tetrahydrocannobinol (THC) level is 5 ng/ml or higher.  If your THC level is below 5ng/ml, you can still be charged with DUI drugs if the officer has probable cause to believe you are impaired. 

If I have a medical marijuana card, can I drive a vehicle after using marijuana?

How do I get home safely if I'm drunk?