Lawmakers in those states looked to policies on drunken driving for cues on how to legislate against driving while high. But the body absorbs alcohol and cannabis in different ways, the study said. While drunkenness directly correlates to alcohol in the bloodstream, cannabis impairment takes place only when THC makes its way into the fatty tissue of the brain.

Regular marijuana users, including those who take the drug medicinally, often show no signs of impairment after using, according to Jolene Forman, a staff lawyer for the Drug Policy Alliance, a drug-reform advocacy group. She also said that marijuana can stay in the blood for hours, days and even weeks after its effects wear off.

As a result, the presence of THC in blood is not a useful indicator of whether the drug is impairing that person’s ability to drive. Furthermore, the study said, “The practical reality of identifying evaluating, arresting and sampling suspected impaired drivers means that the THC concentration measured in the blood specimen reflects neither the concentration in the subject’s blood at the time of arrest, nor the concentration of active drug in the brain.”

In Montana, Washington, Pennsylvania, Ohio and Nevada, drivers are presumed guilty if they have a certain amount of THC in their blood. Colorado also uses a threshold to assess impairment, though it allows suspects to provide evidence at trial that they were not impaired.

The AAA study recommended that laws relying on thresholds be tossed out and that other factors be used. Mr. Nelson said that, ideally, those would include the failure of a standard field sobriety test and the results of a drug assessment conducted by a trained specialist. That assessment would include a blood test to confirm whether cannabis (or any other drug) was present in the bloodstream.