Although marijuana users can perform simple tasks well while they are high, brain imaging has shown that they have to use more of their brain to do so. Their reaction times are slower, peripheral vision is decreased and multitasking impeded. As a result, when sudden or surprising things occur to complicate those tasks -- such as when a pedestrian stepping in front of a car -- they cannot respond as well. On the other hand, marijuana users tend to be aware they are impaired and try to compensate for it.
It's not clear how much marijuana use contributes to crash risk. Some studies have found that marijuana can double crash risk, but others have found virtually no increase.