Drugs (or alcohol)? There are so many more failure modes that drugs are the least of my concerns. Especially of unspecified type. I'm not the least bit worried about drivers hopped up on tylenol. Humans get distracted while driving, by texting, or simply boredom and start daydreaming. Don't forget about driving while tired. Or emotionally disturbed (divorce or a death; road rage). Human vision systems are also pretty frail and have bad failure modes, eg the sun is close to the horizon and the driver is headed towards the sun.