In a scientifical point of vue, 15 volunters is not a big number. An other question, where come from this volunters : i doubt that i personaly accept to take marijuana to pass a test, i can drink a little, but i refuse to be drunk or to smoke pot .
A very small amount of alcoohol have a paradoxal effect on driving : it improve it due to the relaxation. 0,15 g/L, above 0,50 the negative effect are coming, and things become more and more dangerous. So seem to work the pot.
The marijuana seems to give a less dangerous way of driving than the alcoohol, but personaly if i think i have drink to much , i reduce my speed, because i do not want to die. Perhaps it's because the way alcoohol works on me, when i drink to much (not very often but if there is very good wines to taste ...i could not resist
) i become asleep, but never violent.