Remember that a DUI is not just for driving under the influence of alcohol, it also includes drugs. Driving under the influence of marijuana is still illegal, either as a misdemeanor or felony, depending on a variety of factors including whether there is an accident.
However, there are significant difficulties in the prosecutions ability to obtain a conviction for a marijuana DUI. A major problem facing law enforcement is the lack of capability to determine how much marijuana a driver has in his system, let alone knowing if the driver is really impaired. In 2011, for example, an Illinois man heading to work was arrested for DUI after finding traces of pot in his system. A year later, the court dropped the charges after finding out that hypoglycemia was to blame.