The risk is quantified in the linked story. Stoned driving is about twice as dangerous as just plain ild sober driving.
Maybe… maybe not… via my previous post here.
Source for quotes below (A very good read and VERY well sourced, IMO):
" … Alcohol at 0.75 g/kg (slightly less than four standard drinks) causes high levels of impairment in psychomotor performance and medium-to-high levels of impairment in such tasks as critical flicker fusion and short-term memory. Alcohol impairs pursuit tracking, divided attention, signal detection, hazard perception, reaction time, attention, concentration, and hand-eye coordination.
Alcohol also reduces the perceived negative consequences of risk-taking, which can increase willingness to take risks after drinking, the amount of risk-taking behavior while driving, even at low alcohol doses, and the incidence of road traffic accidents while driving drunk.
…
Surprisingly, given the alarming results of cognitive studies, most marijuana-intoxicated drivers show only modest impairments on actual road tests. Experienced smokers who drive on a set course show almost no functional impairment under the influence of marijuana, except when it is combined with alcohol.
Many investigators have suggested that the reason why marijuana does not result in an increased crash rate in laboratory tests despite demonstrable neurophysiologic impairments is that, unlike drivers under the influence of alcohol, who tend to underestimate their degree of impairment, marijuana users tend to overestimate their impairment, and consequently employ compensatory strategies.
Cannabis users perceive their driving under the influence as impaired and more cautious, and given a dose of 7 mg THC (about a third of a joint), drivers rated themselves as impaired even though their driving performance was not; in contrast, at a BAC 0.04% (slightly less than two “standard drinks” of a can of beer or small 5 oz. glass of wine; half the legal limit in most US states), driving performance was impaired even though drivers rated themselves as unimpaired. Binge drinkers are particularly likely to rate themselves as unimpaired, possibly because they tend to become less sedated by high doses of alcohol. … "
