I could be mis quoting some things, but there are different THC metabolites.

The THC-COOH stays in your bloodstream only for a short time after smoking. 4 hours after consumption i think is a time frame I saw somewhere.

The THCa is what they find during drug testing that remains in your system for a very long time.

Read through some of these studies to get a grasp on Marijuana and Driving.
Marijuana And Actual Driving Performance