Concentration Is Not Impairment: THC Blood Levels in California Driving Cases
Law + Science = Forensics
Cannabis and Driving
For alcohol, concentration and effect track each other closely enough to legislate. For cannabis they come apart almost immediately, and no California statute pretends otherwise.
California prosecutes cannabis driving cases under Vehicle Code section 23152, subdivision (f), which requires proof that the driver was actually under the influence. There is no numerical threshold for THC comparable to the alcohol standard. That is not an oversight or a gap waiting to be filled. It reflects a pharmacological reality that has defeated every attempt to write a defensible number.
The curve runs backward from what juries expect
Inhaled THC reaches peak plasma concentration during or within minutes of smoking, then falls steeply as it redistributes into tissue. Whatever behavioral effect occurs tends to peak later, while the blood level is already dropping. The two curves are offset. By the time a driver is stopped, transported, and drawn, often well over an hour later, the blood concentration is on a completely different part of its trajectory than any effect would be.
This is why a low number cannot be read as proof of sobriety and a high number cannot be read as proof of impairment. The relationship between the two is not stable across time, and it is not stable across people.
The prosecution’s expert will usually concede that concentration does not establish impairment, and then testify to impairment anyway based on the officer’s observations. Separating those two things is the work.
The draw time is the fulcrum. Establish the interval between driving and collection, then ask the witness to describe where on the concentration curve the driver was at the moment of driving. There is no honest answer that helps the prosecution.
Frequent use changes everything about the number
THC is lipophilic and accumulates in tissue with repeated exposure. Regular users can carry measurable blood THC well beyond any window of effect, and can show detectable levels after periods of abstinence that no one would describe as recent use. A single measurement in a frequent user carries essentially no information about when that person last used, let alone whether they were affected while driving.
Carboxy THC, the inactive metabolite, is worse still as an impairment indicator. It is not psychoactive, it persists far longer than the parent compound, and its presence establishes prior exposure and nothing else. When a report leads with a metabolite, the report is telling you about history, not about the drive.
What the crash risk data show
The most frequently cited work on this question is the federal case control crash risk study published by the National Highway Traffic Safety Administration in 2015. The unadjusted numbers showed an association between THC positive drivers and crash involvement. After adjustment for age, gender, race, and alcohol, the odds ratio for THC positive drivers came out at approximately 0.94, which is to say no statistically significant elevated crash risk once the confounders were accounted for. The same study found a steep, unmistakable dose response for alcohol.
The comparison is the point. When a method can detect the alcohol signal clearly and cannot detect a THC signal at all, the problem is not the sample size.
Questions a THC number cannot answer
- When did this person last use cannabis.
- What was the concentration at the time of driving, as opposed to the time of the draw.
- Is this person a frequent user carrying residual levels.
- What was the route of administration, which determines the entire shape of the curve.
- Was this person affected while driving, which is the only question the statute asks.
Where the real fight happens
Because the number cannot carry the case, cannabis prosecutions lean on officer observation, field sobriety testing, and drug recognition evaluations. Each of those has its own evidentiary problems, which we address separately in our work on drug identification and interpretation. The toxicology’s role in a cannabis case is usually not to prove anything affirmative. It is to establish how little the chemistry is capable of proving, so the jury evaluates the observational evidence on its own merits rather than as a confirmation of a number.
Questions we are asked about this
Does California have a legal THC limit for driving?
No. Cannabis driving cases are prosecuted under Vehicle Code section 23152, subdivision (f), which requires proof of actual impairment. Proposals for a numerical threshold have not become law, and the pharmacology is the reason.
How long does THC stay in blood?
It depends heavily on use frequency. Occasional users clear the parent compound relatively quickly, while frequent users can show measurable blood THC long after any effect has ended. The inactive metabolite persists far longer still.
Can a blood test show when someone last used cannabis?
Not reliably from a single sample. The concentration at any moment depends on the route, the dose, the time elapsed, and the person’s use history, and a single measurement cannot separate those variables.
Sources and further reading
- California Vehicle Code section 23152. Text.
- Compton RP, Berning A. Drug and Alcohol Crash Risk. National Highway Traffic Safety Administration, Traffic Safety Facts Research Note, 2015.
- Peer reviewed literature on THC pharmacokinetics, tissue distribution, and the offset between blood concentration and measured effect. Search on PubMed.
This article is general information about forensic science and California law. It is not legal advice, and reading it does not create an attorney client relationship.
