In very one of these cases, its revealed that the driver also had unspecified levels of booze, tranquilizers and/or opiates. 
People on opiates just nod off, and fall asleep while driving,  and tranquilizers impair driving nearly as much as alcohol. 

While cannabis also causes impairment, the effects are much less severe than booze or pills. Since THC has a half-life of about 30-45 minutes, impairment related to THC generally dissipates much quicker than impairment related to alcohol or pills.   

Driving impaired is by definition dangerous and irresponsible, but it  requires a credible impairment levels.  In 1993, the NHTSA studied the effects of THC on driving impairment, finding "a moderate THC dose of 100 ng/mg" has minor impairment effects,  and as a consequence,   "THC's adverse effects on driving appear relatively small".  

Doesn't it seem odd that the impairment level for THC dropped from 100 ng down to 5 ng, remained constant for  alcohol.