Driving While Stoned

THC is the intoxicant most commonly detected in US drivers, with approximately 13% of drivers testing positive for marijuana use, compared to the 8% that show a measurable amount of alcohol (NHTSA, 2015). (The two figures are not strictly comparable because cannabis remains detectable for much longer than alcohol, and also for long after the driver is no longer impaired; therefore, the difference in rates does not show that stoned driving is more common than drunk driving.) Cannabis intoxication has been shown to impair reaction time and visual-spatial judgment. 

Many states, including those where cannabis sales are now permitted by state law, have laws against cannabis-impaired driving based on the drunk-driving model, defining criminally intoxicated driving as driving with more than a threshold amount of intoxicant in one’s bloodstream—a per se standard—as opposed to actual impairment. That approach neglects crucial differences between alcohol and cannabis in their detectability, their pharmacokinetics, and their impact on highway safety. 
Cannabis intoxication is more difficult to reliably detect chemically than alcohol intoxication. A breath alcohol test is (1) cheap and reliable; (2) sufficiently simple and non-invasive to administer at the roadside; and (3) a good proxy for alcohol in the brain, which in turn is (4) a good proxy for subjective intoxication and for measurable driving impairment. In addition, (5) the dose-effect curve linking blood alcohol to fatality risk is well-established and steep.

None of those things is true for cannabis. A breath test remains to be developed. Oral-fluid testing can demonstrate recent use but not the level of impairment. A blood test requires a trained phlebotomist and therefore a trip to a medical facility, and blood THC levels drop very sharply over time-periods measured in minutes. Blood THC is not a good proxy either for recency of use or for impairment, and the dose-effect curve for fatality risk remains a matter of sharp controversy. The maximum risk for cannabis intoxication alone, unmixed with alcohol or other drugs, appears to be more comparable to risks such as talking on a hands-free cellphone (legal in all states) than to driving with a BAC above 0.08, let alone the rapidly-rising risks at higher BACs. Moreover, the lipid-solubility of THC means that a frequent cannabis user will always have measurable THC in his or her blood, even when that person has not used recently and is neither subjectively intoxicated nor objectively impaired. That suggests criminalizing only combination use, while treating driving under the influence of cannabis (however this is to be proven) as a traffic offense, like speeding.