A prosecutor blamed the Beer crash on "speed and weed," but a jury that heard expert testimony on marijuana's effects at his trial deadlocked on a homicide charge and other felonies related to whether the teenager was impaired by marijuana. Studies of marijuana's effects show that the drug can slow decision-making, decrease peripheral vision and impede multitasking, all of which are important driving skills. [...] combining marijuana with alcohol appears to eliminate the pot smoker's exaggerated caution and to increase driving impairment beyond the effects of either substance alone. Dr.