News AAA Study Concludes Marijuana DUI Limits are Unreliable

An AAA Foundation for Traffic Safety (American Automobiles Association) study concluded that (1) the specific "per se" legal limits for the presence of marijuana (THC) as per DUI laws are inherently unreliable without additional supporting evidence; and (2) the number of fatalities involving drivers who recent used marijuana doubled in Washington State, which recently legalized recreational marijuana usage. In addition to seriously questioning the correlation of THC testing of drivers with actual impairment, the AAA also warned states to consider the potential impact of the legalization of marijuana and cannabis products. But it also alluded to the fact that unsafe drivers may be freed while others may be wrongly convicted.

AAA Releases Results of Four Efforts Analyzing Cannabis Usage

The AAA recently published "Impaired Driving And Cannabis" which included six hyperlinks on its website to several studies and research on traffic safety. In addition to an overview on the impact of alcohol and marijuana, the AAA studies included:
  • Prevalence of Marijuana Use among Drivers in Washington State
  • An Evaluation of Data from Drivers Arrested for Driving Under the Influence in Relation to Per se Limits for Cannabis
  • Cannabis Use among Drivers Suspected of Driving Under the Influence or Involved in Collisions: Analysis of Washington State Patrol Data
  • Driving Under the Influence of Alcohol and Marijuana: Beliefs and Behaviors, United States, 2013-2015

Marijuana Study Results Leave Questions, Few Easy Answers

DUI and DWI laws which criminalize driving under the influence of cannabis in Colorado, Washington, and Montana set a quantitative threshold for concentration of delta-9-tetrahydrocannabinol (THC) in the blood. THC, the active component of cannabis, in a person's blood. A driving under the influence of cannabis charge (DUI) requires blood THC concentrations above or below 5 ng/mL, with lower results potentially resulting in violations such as Driving While Ability Impaired (DWAI) in Colorado. The AAA studies used the 5ng/mL THC level in their tests.

A DUI "per se" usually refers to a threshold of concentration in the blood which, if exceeded, would result in a DUI conviction regardless of whether the driver was actually impaired. So if a driver was tested with a 0.08 BAC (Blood Alcohol Concentration), they would be guilty of a DUI per se and evidence that they may not have been impaired would not be relevant to a conviction. In certain states, the presence of 5 ng/mL of THC does not result in a DUI per se although it does presume that the driver was impaired as a result of cannabis usage or consumption.

Some of the conclusions reached were:
  • The typical sobriety indicators were significantly greater incidence in cannabis-positive persons (red, bloodshot and watery eyes, eyelid tremor, lack of convergence, and rebound dilation).
  • Subjects tested with THC concentrations from 1-10 ng/mL did not reveal a reliable threshold level of THC concentration which would provide an acceptable level of agreement with the Standardized Field Sobriety Test for a per se limit.
  • 70 percent of cannabis-positive drivers arrested for DUI had THC concentrations below 5 ng/mL.
  • The impact of driver impairment is significantly greater when the presence of THC also includes alcohol or other drugs.
  • THC-involved driving is becoming relatively common and appears to be increasing in the state of Washington.
The AAA recommended against laws relying solely on specific THC limits, such as those in Colorado, Montana, Ohio, Pennsylvania and Washington states. They suggested that greater accuracy and fairness would be achieved through the use of specially trained police officers performing a battery of driver impairment tests along with a general confirmation of a recorded presence of THC. Screening for indications of drug use would include the indicators of a Standardized Field Sobriety Test and drug use such as pupil dilation, tongue color and exhibited behavior.

How does this Impact my DUI Case?

There appears to be a growing amount of evidence to suggest that convictions for marijuana based DUI charges based solely on THC levels are inherently unfair. Frequent users of cannabis can exhibit higher levels of THC in contrast to occasional users whose THC levels will decline more rapidly. But it's too early to predict the impact of the AAA study concerning cannabis laws. States that have zero tolerance DUI laws do not require evidence of actual impairment since the mere presence of a positive test is sufficient to convict a DUI suspect. However, it may make it more difficult for a prosecutor to prove that a driver was impaired if the primary evidence consists of THC levels and the law requires more than a specific threshold.
Legal Practice
Crime - DUI & DWI
  1. Colorado
  2. Montana
  3. Nevada
  4. Ohio
  5. Pennsylvania
  6. Washington
About author
Michael Wechsler
Michael M. Wechsler is an experienced attorney, founder of, A. Research Scholar at Columbia Business School and of-counsel to Kaplan, Williams & Graffeo, LLC. He was also an SVP and chief Internet strategist at and legal consultant at Kroll Ontrack, a leading service e-discovery and computer forensics service provider.


There are no comments to display.

Article information

Michael Wechsler
Article read time
3 min read
Last update

More in Traffic & Motor Vehicles Law

More from Michael Wechsler

Share this article