>> a reduced ability to divide one's attention. >> reporter: marijuana advocates argue marijuana is less debilitating than alcohol. but a new study shows those who drive within 3 hours of smoking pot are at a greater risk of causing a crash. >> it inhibits the ability to jump the judge time and distance. >> reporter: in state where marijuana is legal, the level is 5 man or grams. the impairment some argue is equal to alcohol. >> it's difficult to determine whether that five nanograms will change. >> reporter: heavy users though not impaired can test