Is Pot or Alcohol more lethal when driving?

A WSJ story (published today) indicates that 40% of recent traffic fatalities tested positive for THC (the active ingredient in Pot, Wright State University).

I did some more research, and the majority of studies (including the data presented by BioMed Central) appear to suggest the following:

• 30-32% of fatalities involve alcohol.

• 33% of fatalities involve cannabinoids.

Bottom line: It's bad to drive impaired, but the cops can't test for Pot yet? BTW, I do not drink and I don't do hooch. I'm no fun at a party and overly wound up.