The numbers do appear quite staggering. It can't just be the dead drivers - there must be similar numbers of stoned drivers who are causing accidents, maybe killing others, while surviving themselves.

As far as driving goes, any amount of drugs or alchohol is going to reduce reactions times, in addition to any impaired judgement or ability to control the vehicle. Even a couple of 1/10ths of a second in increased reaction time is enough to make the difference between braking in time and hitting another car or pedestrian/etc.