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 ....