In most states, being “under the influence” means that the driver is incapable of driving safely due to the effects of drug or alcohol use.
As you are probably aware, when it comes to alcohol, a blood alcohol level of 0.08 percent of the driver’s blood, by volume, will conclusively establish that the driver is under the influence (if the level is less, the prosecutor can still point to the driver’s actions to prove that he was under the influence). In some states, the blood alcohol level threshold is even lower if the driver is a minor.
When marijuana is involved, however, states have different approaches to establishing that the driver was under the influence, as shown below.
In states with so-called “per se” DUI laws, any amount of marijuana in the driver’s system at the time of the offense will conclusively establish impairment. In these states, a prosecutor will not need to present any further evidence (such as behavior consistent with being under the influence or unsafe driving) in order to establish that the driver was under the influence.
State per se laws often include marijuana metabolites—compounds left over when the body metabolizes (or processes) marijuana—which can remain in a person’s body for days, weeks, or longer after marijuana use. While metabolites indicate that the person ingested marijuana at some point in the past, they do not indicate how long ago, or necessarily point to current impairment. Even so, state per se laws that include metabolites accept their presence as conclusive evidence of impairment for the purposes of a DUI charge.
As they do with blood alcohol thresholds, some states consider a level of marijuana (or marijuana metabolites) in the driver’s blood or urine—usually in nanograms/ liter—as conclusive proof of impairment. As with per se laws, the prosecutor will not need to prove that the driver’s senses were impaired—no need for field sobriety test results, or testimony about the driver’s speech, balance, or poor driving.
In these states, having a concentration level that’s lower than the threshold does not necessarily mean that the driver was not under the influence, however. The prosecutor may still point to the driver’s actions and behavior (such as his driving) to show that the driver was under the influence.
In the minority of states, the prosecutor must always establish that the driver was behaving in a way that showed that he was under the influence of marijuana at the time of the arrest—regardless of (even relatively high) marijuana blood or urine concentration levels. Prosecutors can do this by showing that the driver had impaired balance or speech, or that he was driving erratically—even that he smelled of marijuana.
People are sometimes surprised to learn that the prosecution need not show actual unsafe driving to prove that the driver was under the influence. Merely being under the influence and driving will suffice. For example, suppose you are involved in an accident that you did not cause—your driving was just fine. But the police officer who comes to the accident scene smells marijuana in your car, observes your reddened eyes and tell-tale behavior, and sees half-smoked joints in the ash tray. This may be enough evidence to charge you with driving while under the influence, even though your driving was not unsafe.