Expert Testimony Regarding Positive Marijuana Tests May Not be Admissible in Motor Vehicle Accident Litigation

As states trend towards legalizing recreational use of marijuana—recreational marijuana use is legal in Alaska, California, Colorado, D.C., Maine, Massachusetts, Nevada, Oregon, and Washington—the need for a reliable test for marijuana intoxication is on the rise, and the lack of such a test poses problems for admitting or excluding evidence that marijuana impaired an individual’s ability to drive. 

The reliability of expert testimony to establish marijuana impairment recently came before the California Court of Appeal in David v. Hernandez, et al., No. B270133 (Cal. Ct. App. July 25, 2017), a case arising out of a motor vehicle accident between a van and a tractor-trailer.  In David v. Hernandez, Plaintiff David filed a complaint against Defendants Hernandez and Hernandez dba D&H Trucking for injuries sustained after his van collided with Defendants’ tractor-trailer.  Plaintiff David admitted to smoking marijuana within 36 hours of the accident and a urine sample collected at the hospital was positive for THC (tetrahydrocannabinol), the psychoactive ingredient in marijuana.  At trial, Defendants, attempting to establish comparative negligence, sought to present expert testimony that Plaintiff David was impaired by marijuana at the time of the accident. 

The trial court excluded the expert testimony finding that the test was preliminary and that the attempt to correlate impairment from that test lacked foundation.  On appeal, the California Court of Appeal affirmed, finding that under Evidence Code 801, the expert testimony was speculative as to THC concentration in Plaintiff David’s system at the time of the accident and as to whether the amount in his system impaired his ability to drive. 

Although Plaintiff David’s test was positive for THC, the Court of Appeal found that the expert testimony was unreliable because it lacked foundation as to the amount of the active type of THC in Plaintiff David’s system.  Marijuana is composed of both active and inactive THC, and only active THC can lead to impaired driving.  Inactive THC, however, stays in the blood system for hours—even days—after the active THC has long left the body.  Because the test did not establish the amount of active THC in Plaintiff David’s system, the Court of Appeal found the expert testimony lacked a reasonable basis for concluding that Plaintiff David was impaired.  Moreover, even if the testimony could establish the level of active THC in Plaintiff David’s system, this testimony was speculative as to “whether the amount was sufficient to impair his ability to drive a motor vehicle.” 

This decision serves as a guidepost for attorneys seeking to exclude or admit evidence of marijuana impairment, and highlights the difficulties marijuana use presents in vehicle accident litigation.  Until there is a reliable method for establishing the level of active THC in a person’s system and a consensus on the level of active THC sufficient to impair driving, courts may remain hesitant to allow expert testimony to establish impairment.  Attorneys seeking to exclude evidence of marijuana impairment should focus on the unreliability of such evidence under David v. Hernandez, while attorneys attempting to establish marijuana impairment should rely upon other evidence, such as a person’s driving pattern, physical appearance, and the presence of marijuana or paraphernalia in the vehicle.