Driving under the influence of marijuana is potentially very dangerous.
This research shows the effects of marijuana negatively impact driving. Here is a quote from the research, “Cannabis smoking increases lane weaving and impaired cognitive function. Critical-tracking tests, reaction times, divided-attention tasks, and lane-position variability all show cannabis-induced impairment. Despite purported tolerance in frequent smokers, complex tasks still show impairment. Combining cannabis with alcohol enhances impairment, especially lane weaving.” 
The federal government should prevent states from decriminalizing marijuana use.