Researchers found nearly 90 per cent of drivers with a blood alcohol level of .05 or more were deemed to have caused the accident after a police investigation. However, drivers with marijuana in their systems were responsible for only 50.8 per cent of the accidents they were involved in, meaning they were only slightly more likely to have caused an accident than somebody with no intoxicant in their system. Psychopharmacologist Dr. Barry Beyerstein of Simon Fraser University in Vancouver, who has been qualified as an expert on the effects of marijuana in court, says the reason for these results lie in the way marijuana affects people. It also impairs motor skills and reaction time. The results of alcohol when driving is clear from the statistics.
Marijuana tends to do the exact opposite to people.
They are more cautious. They tend to overcompensate for their impairment. Where the danger with marijuana comes in, is it affects people's attention. They'll be driving and a old song will come on, and they stop concentrating on the road. The lapse in attention is the danger when driving after ingesting marijuana.