Arizona THC Marijuana DUI Defense: How Federal NHTSA Reports Can Help Win Your Case
Quick answer: Since Arizona voters passed Proposition 207 in 2020, there is no automatic (“per se”) blood THC limit for a marijuana DUI in Arizona. The State must prove actual impairment beyond a reasonable doubt. Because federal research — including the National Highway Traffic Safety Administration’s (NHTSA) own 2017 report to Congress — finds no reliable scientific link between THC blood levels and impairment, Arizona DUI defense attorneys can use the State’s own expert witness to read NHTSA’s findings into evidence under Arizona Rule of Evidence 803(18), creating reasonable doubt in front of the jury.
How Proposition 207 Changed Arizona Marijuana DUI Law
In November 2020, Arizona voters passed Proposition 207 (the Smart and Safe Arizona Act), legalizing adult recreational marijuana use statewide. Beyond legalizing possession, Prop 207 fundamentally changed how Arizona marijuana DUI trials work.
Before Prop 207, a defendant charged with driving with THC in their system under A.R.S. § 28-1381(A)(3) had to prove they held a valid medical marijuana card and that they were not impaired. The burden was on the defendant. Today, Prop 207 added A.R.S. § 36-2852(B), which says a person with marijuana metabolites in their body is only guilty of a DUI if they are also impaired “to the slightest degree.” This flips the burden back where it normally belongs: the State must now prove impairment beyond a reasonable doubt, whether or not the driver has a medical card.
Why There Is No “Legal Limit” for THC in Arizona
Unlike alcohol, which has a clear .08% blood alcohol concentration (BAC) legal limit, Arizona has no equivalent per se limit for THC. Section 7.2 of Prop 207 (page 16) specifically states that no per se THC limit can be set until scientific research conclusively shows that all people are impaired at a particular blood THC level. That conclusive scientific evidence does not exist. As a result, Arizona prosecutors cannot simply point to a number on a blood test and call it proof of guilt the way they can with alcohol.
This makes Arizona THC DUI trials fundamentally different from alcohol DUI trials. Instead of a bright-line number, the jury must weigh all the evidence — police observations, field sobriety tests, video, and importantly, scientific evidence — to decide whether the driver was actually impaired at the time they were driving.
Turning the State’s Own Expert Witness Into a Defense Tool
In most Arizona THC DUI trials, the prosecution calls an expert witness — often a Drug Recognition Expert (DRE) or toxicologist — to testify about the science of marijuana impairment and argue it supports a conviction. What many drivers don’t realize is that the same expert can be used by the defense to do the opposite.
Arizona Rule of Evidence 803(18) allows statements from a treatise, periodical, or pamphlet to be read into evidence as an exception to the hearsay rule, so long as the publication is established as a reliable authority — either by the expert’s own admission, by another expert’s testimony, or by judicial notice. The Arizona Court of Appeals confirmed in State v. West, 238 Ariz. 482, ¶ 70, that it is the publication as a whole — not each individual article — that needs to be established as reliable.
Virtually every State DUI expert in Arizona agrees, on the record, that NHTSA is a reliable government authority on impaired driving. Once that agreement is on the record, defense counsel can read NHTSA’s own published findings to the jury, in the expert’s own voice.
What NHTSA’s 2017 Report Actually Says About THC and Impairment
The NHTSA’s 2017 report to Congress, Marijuana-Impaired Driving: A Report to Congress (DOT HS 812 440), reviewed the existing scientific literature on marijuana and driving and reached conclusions that directly undercut the State’s typical theory of the case. Among them:
- Studies that have been conducted do not show that the level of THC in the blood and the degree of impairment are closely related (Page 12).
- There are currently no evidence-based methods to reliably determine marijuana-impaired driving from blood THC levels alone (Page 18).
- The poor correlation between blood THC concentration and impairment is part of why NHTSA itself concluded that setting a per se THC limit “is not meaningful.”
Because this is the federal government’s own published research — not a defense expert hired for trial — it carries significant weight with jurors. Hearing a prosecution witness concede, under Rule 803(18), that the federal government has found no reliable scientific link between THC levels and impairment is often more persuasive coming from the State’s own witness than it would be coming from the defense.
Why This Matters If You’re Facing a THC DUI Charge in Arizona
If you were arrested for a marijuana DUI anywhere in Arizona — Phoenix, Scottsdale, Tempe, Mesa, Glendale, Chandler, or elsewhere in Maricopa County or statewide — the State cannot rely on a blood test number alone. They must prove, beyond a reasonable doubt, that you were impaired to the slightest degree at the time you were driving. Federal research gives defense attorneys a powerful, credible tool to challenge that proof using the prosecution’s own expert.
This is not a do-it-yourself strategy. Successfully using Rule 803(18) and NHTSA’s findings at trial requires knowing how to lay the foundation with the State’s expert on cross-examination, timing the request correctly, and anticipating the prosecutor’s objections. An experienced Arizona DUI defense attorney can evaluate your blood test results, your arrest report, and the specific expert the State intends to call, and build a defense strategy around it.
Frequently Asked Questions About Arizona THC DUI Defense
Is there a legal THC limit for driving in Arizona?
No. Unlike alcohol’s .08% BAC limit, Arizona has no per se or automatic THC limit for marijuana DUI cases. Proposition 207 specifically prohibits setting one until science conclusively shows all people are impaired at a specific level, and no such conclusive evidence currently exists.
Can I be convicted of a DUI in Arizona just for having THC in my blood?
Not anymore. Since Proposition 207 took effect in 2020, having marijuana metabolites in your system is not enough on its own. The State must prove you were impaired “to the slightest degree” at the time you were driving, under A.R.S. § 36-2852(B) and § 28-1381.
Does having a medical marijuana card protect me from a DUI charge?
A medical marijuana card does not exempt you from DUI laws, and you can still be charged if you’re impaired. However, since Prop 207, the practical burden of proof for THC presence is the same for medical and recreational users: the State must prove actual impairment.
What is Arizona Rule of Evidence 803(18) and how does it help my defense?
Rule 803(18) is a hearsay exception that allows statements from a reliable published source, like a federal government report, to be read into evidence once an expert witness agrees the source is authoritative. In THC DUI cases, this can allow defense attorneys to introduce NHTSA’s own findings about the lack of a reliable link between THC levels and impairment, using the State’s own expert witness.
What does the NHTSA 2017 report say about marijuana and driving impairment?
NHTSA’s 2017 report to Congress found that studies do not show a close relationship between THC blood levels and the degree of impairment, and that there are currently no reliable evidence-based methods to determine marijuana-impaired driving from THC concentration alone.
Do I need a lawyer for a THC DUI charge in Arizona?
Yes. THC DUI cases turn heavily on scientific and expert testimony, and effectively using tools like Rule 803(18) requires legal experience with cross-examination, evidentiary foundations, and Arizona DUI case law. An experienced Arizona DUI defense attorney can review your specific blood test results and arrest circumstances to determine the strongest defense.
About the Author: Gordon Thompson is a Phoenix, Arizona DUI and criminal defense attorney. Watch his video explainer: NHTSA Reports and THC DUI Defense.
For more information about Arizona DUI and criminal defense issues, contact Gordon Thompson, who draws on his experience defending Arizona DUI cases to write a blog on topics of interest to the people he represents.
Website: www.GordonThompsonAttorney.net
Blog: www.GordonThompsonAttorney.net/blog