The Complex Cannabis DUI

Most people think of DUIs as related to being under the influence of alcohol, but the scope of DUIs is much broader than that, and California is still trying to figure out how to uphold a legal limit when drivers are under the influence of cannabis. If you get pulled over in California and blow a 0.08, the case against you writes itself. The number does the talking. Marijuana doesn't work that way, and prosecutors, defense attorneys, and honestly the scientists studying this stuff are still arguing about why.

Alcohol DUIs run on a straight line. Under Vehicle Code 23152(b), a blood alcohol concentration of 0.08% or higher is per se impairment. The prosecution doesn't have to prove you were driving badly or slurring your speech, just that you exceeded the number. Cannabis DUIs are prosecuted under a different subsection, 23152(f), which criminalizes driving "under the influence of any drug." The lack of a number attached or threshold defined leaves prosecutors struggling to prove impairment the old-fashioned way.

Putting a Threshold Number on THC

Lawmakers have tried to borrow the alcohol model and slap a THC threshold on cannabis more than once. It keeps falling apart, and not because nobody's tried hard enough. THC doesn't metabolize as alcohol does, making putting a hard threshold line or number very challenging for lawmakers.

Alcohol enters the bloodstream, peaks, and clears out at a fairly predictable rate that correlates reasonably well with impairment. The challenge with THC is that it is fat soluble and can linger in the blood and show up on a test days, and even weeks after the last time they used cannabis. Proving impairment is difficult when it remains in the bloodstream for so long, well after its effects have passed. The other challenge is that frequent cannabis users build up a tolerance that doesn’t have the same impairment as someone who doesn’t have a tolerance built up.

Some states, like Colorado, have tried to implement a number (5 ng/mL), but it doesn’t apply neatly when prosecution is trying to prove impairment. This challenge other states are facing is one of the main reasons that California hasn't adopted one, and why groups that have studied it, including the state's own task forces, have generally recommended against it.

How Prosecutors Approach Cannabis Arrests

When there is no specific number to use as a baseline, cannabis influence cases become a patchwork approach using circumstantial evidence. The resources used include:

• Officer observations at the stop: driving pattern, odor, red eyes, delayed reactions
• Field sobriety tests, which were designed with alcohol in mind and don't translate as neatly to cannabis. A toxicology can show the presence of THC, but that only proves it has been used, not that the driver was impaired at the time of arrest.
• Trained officers in the field may administer Drug Recognition Expert (DRE) evaluations

The challenge for prosecutors in California is that every one of the above sources is contestable, which is exactly what defense attorneys focus on. A positive blood test tells a jury someone used cannabis at some point, but not that the driver was impaired at the time of arrest. This gap can create enough doubt that in cross-examination, the findings are picked apart because the science can and can't actually support it.

A Missing Per Se Limit Is Arguable

The absence of a per se limit sounds like it should make cannabis DUIs easier to beat, and in some cases it is. The other side is that each case hinges heavily on subjective judgment calls made by officers who may have limited training in recognizing cannabis impairment specifically, as opposed to alcohol impairment, which most patrol officers see constantly.

The subjectivity in these cases can work for and against someone facing DUI charges for cannabis. It can work in a defendant's favor when the evidence is easily picked apart but can also work against them when an officer's field notes are detailed and consistent. Field sobriety tests, including when the blood draw occurs relative to the stop, and whether a DRE evaluation was even conducted, all end up mattering more in cannabis cases than they typically do in a straightforward alcohol DUI.