Further, in Willey v. Williamson Produce, 357 N.C. 41, 577 S.E.2d 622, 2003 N.C. LEXIS 309 (2003)  per curiam adopting dissent in Willey v. Williamson Produce, 149 N.C. App. 74  ), an industrial commission case, the Court noted the distinction between the analytically significant levels of a metabolite and pharmacologically significant levels.  The case noted that marijuana metabolites can be found as long as 20 days after ingestion.  The case further noted the pharmacologic level of cocaine has been determined to be 300 ng per ml of urine.  The case did not indicate the pharmacologically relevant level of marijuana metabolites. 

I reviewed the medical literature quickly and found the following in google scholar: 

Marijuana, Alcohol and Actual Driving Performance



Hum. Psychopharmacol. Clin. Exp. 15, 551±558 (2000) 

Experimental Psychopharmacology Unit, Brain & Behavior Institute, Maastricht University, The Netherlands Web

The objective of the current study was to assess the separate and combined e€ects of marijuana and alcohol on actual driving performance. Eighteen subjects were treated with drugs and placebo according to a balanced, 6-way, crossover design. On separate evenings they were given weight calibrated D9-tetrahydrocannabinol (THC) doses of 0, 100 and 200 mg/kg with and without an alcohol dose su cient for achieving blood alcohol concentrations (BAC) of 0_04 g/dl while performing a Road Tracking and Car Following Test in normal tra c. Main outcome measures were standard deviation of lateral position (SDLP), time driven out of lane (TOL), reaction time (RT) and standard deviation of headway (SDH). Both THC doses alone, and alcohol alone, signi®cantly impaired the subjects performances in both driving tests. Performance de®cits were minor after alcohol and moderate after both THC doses. Combining THC with alcohol dramatically impaired driving performance. Alcohol combined with THC 100 and 200 mg/kg produced a rise in SDLP the equivalent of that associated with BAC.0_09 and 0_14 g/dl, respectively. Mean TOL roseexponentially with SDLP. Relative to placebo mean RT lengthened by 1_6 s under the combined in¯uence of alcohol and THC 200 mg/kg. Changes in SDH ranged between 0_9 and 3_8 m. Low doses of THC moderately impair driving performance when given alone but severely impair driving performance in combination with a low dose of alcohol.

Copyright # 2000 John Wiley & Sons, Ltd. 


There is no doubt that D9-tetrahydrocannabinol

(THC) impairs its users' cognitive and psychomo-

tor abilities to an extent largely determined by the

inhaled or ingested dose. It is also certain that the

dose preferred by cannabis smokers, i.e. around

300 mg/kg, is su cient for impairing performance

in potentially dangerous tasks such as driving

(Robbe, 1994). It is less certain that those doses

cause degrees of impairment that seriously com-

promise driving ability.

 Also see


intoxication by marijuana


In the NEJM study, drivers were stopped for reckless driving and then given tests.  However, the intoxication was not measured by the standardized FST. Finally, a metastudy indicates that impairment does not occur if the level of active metabolites is below 10 ng/ml blood 

Grotenhermen F., Leson G., Berghaus G., Drummer O. H., Kruger H.-P., Longo M. et al. Developing limits for driving under cannabis. Addiction 2007; 102: 1910-17. 


Objective  Development of a rational and enforceable basis for controlling the impact of cannabis use on traffic safety.

Methods  An international working group of experts on issues related to drug use and traffic safety evaluated evidence from experimental and epidemiological research and discussed potential approaches to developingper se limits for cannabis.

Results  In analogy to alcohol, finite (non-zero) per se limits for delta-9-tetrahydrocannabinol (THC) in blood appear to be the most effective approach to separating drivers who are impaired by cannabis use from those who are no longer under the influence. Limited epidemiological studies indicate that serum concentrations of THC below 10 ng/ml are not associated with an elevated accident risk. A comparison of meta-analyses of experimental studies on the impairment of driving-relevant skills by alcohol or cannabis suggests that a THC concentration in the serum of 7-10 ng/ml is correlated with an impairment comparable to that caused by a blood alcohol concentration (BAC) of 0.05%. Thus, a suitable numerical limit for THC in serum may fall in that range.

Conclusions  This analysis offers an empirical basis for a per se limit for THC that allows identification of drivers impaired by cannabis. The limited epidemiological data render this limit preliminary. 

Bottom line I think Clark is correct with respect to the State having to prove impairment, not just drug concentration.  An expert may try and say that the amount present is above the amount the literature reports causes impairment.  However, you are going to have to get someone to give you the what your actual values are (ng/ml or some other amount). 

Dean P. Loven