With New Year’s Eve just hours away, the likelihood of law enforcement roadside checks for driving impairment will be spiking. And the question arises as to what are the per se laws when it comes to cannabis and in particular, THC and its metabolites. Unfortunately, legal limits for THC in blood or plasma have largely been based on what is the technically achievable limit of detection (LOD), making the legal limit a function of its detectability rather than the impairment caused by it, and as for alcohol, a driver is assumed impaired if he or she exceeds that limit. The LOD for THC in blood serum is about 1 ng/mL and that is actually the legal limit under German law since 1999. Such strict, non-science based laws, simplifies law enforcement but does not only punish impaired drivers. And measuring for the presence of the slowly dissipating THC metabolite, THC-COOH, in blood reflects the use of cannabis in the past days or weeks and certainly is not indicative of impairment.
While the parallel of dose increasing impairment exists between THC and alcohol, the variability between THC dose and individuals varies greatly. This variability arises from the inherent tolerance an individual develops to THC, the mode of exposure for combustion and the rate of absorption for oral ingestion and no doubt specific, yet to be identified genotypic variants across the human population that we already know exist for other major drugs used to affect the central and peripheral nervous systems.
Another approach to setting impairment levels for THC was to compare it in a dose equivalency study with alcohol. Here Berghaus determined that a blood alcohol content of 0.073% corresponded to 50% impairment and the corresponding equivalent plasma THC level was 11 ng/mL, keeping in mind that the concentration in plasma is roughly twice that in whole blood.
Meanwhile the National Highway Traffic Safety Administration acknowledges that: “It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects. Concentrations of parent drug and metabolite are very dependent on pattern of use as well as dose.” 
Plasma THC levels fall below 5 ng/mL in less than 3 hours after smoking but that plasma elimination half-lives are best estimated several days after smoking marijuana. THC is primarily metabolized to 11-hydroxy-THC which has equipotent psychoactivity. The 11-hydroxy-THC is then rapidly metabolized to the 11-nor-9-carboxy-THC (THC-COOH) and it is not psychoactive. Chronic users can have mean plasma levels of THC-COOH of 45 ng/mL, 12 hours after use; corresponding THC levels are, however, less than 1 ng/mL. It is illogical to try and predict effects based on blood THC concentrations alone, and currently impossible to predict specific effects based on THC-COOH concentrations.
Little has changed since a 2005 report published by the Journal of the American Medical Association when an expert panel on drugs and driving concluded: “Except for ethanol, determinations of drug concentrations in body fluids are at present of limited value for establishing driving impairment. … Although psychoactive drugs are those most commonly thought to cause impaired driving, their mere presence in body fluids cannot be construed as evidence of impairment.”
Despite what the experts have to say, the issue of what level of THC in one’s blood connotes impairment is an ongoing debate that is being played out in the courts. Specifically, in Colorado where as in most states, the legal limit for impairment for marijuana is 5 ng THC per mL of blood, clearly that limit is presumption only and challengeable in court. In fact, a Colorado jury has already rejected this “standard” for marijuana impairment involving one medical marijuana patient; who was pulled over for an expired license plate, cop smelled marijuana, individual works in a dispensary and had a MMJ card and she passed the roadside sobriety test twice! Her blood test found THC at 19 ng/mL but the jury was unconvinced that the driver in question was impaired and the judge threw the case out.
 Behavioral Effects of Alcohol and Cannabis: Can Equipotencies be Established? 1995 by H.-P. Krüger and G. Berghaus, Center for Traffic Sciences, University of Würzburg, Röntgenring 11, D-97070 Würzburg, Germany.
 JAMA (1985) Developing Science-Based Per Se Limits for Driving under the Influence of Cannabis (DUIC)