One of the greatest concerns held by those who oppose cannabis legalization is the possibility that accidents linked to drugged driving will rise. Now that New Mexico has legalized recreational cannabis, more and more people are beginning to ask if it will make our streets unsafe.

New Mexico’s problems with driver intoxication have, unfortunately, marred much of its history. The state has consistently been one of the worst in the nation for drunk driving deaths. The number of alcohol-related vehicle deaths dropped dramatically last year due to COVID restrictions and lower numbers of drivers on the road, but it interrupted a serious streak. The year before, New Mexico ranked fifth worst in the nation for drunk driving fatalities. In 2018 it was ranked fourth in the nation.

So we can be forgiven for being a little nervous about legalizing a drug that could potentially increase those numbers.

The Testing Problem

But here’s the monkey wrench: There is no widely-accepted test being used by police that can accurately tell if a person is high on cannabis. Many police departments across the nation are forced to test saliva, urine or blood samples to detect marijuana use—all of which have problems all their own.

Urine tests, while excellent at detecting cannabis use, introduce the problem of timing. These tests look for 11-nor-9-carboxy-delta-9-THC (THC-COOH), a metabolite produced when delta-9-THC is broken down in the body. These metabolites can show up in urine for up to 90 days, and the test is unable to determine when the consumption actually took place. The very idea that a joint could effect a driver 90 days later is completely laughable. But as far as the test can tell, there’s no difference between that joint or one that was smoked five minutes ago.

Blood tests are a little better, in that they generally detect THC and THC-COOH only within around 24 hours of the last consumption. But heavy users have been known to test positive even after seven days of abstaining.

The saliva test involves swabbing the inside of a driver’s mouth to detect the presence of a number of drugs (not just cannabis). This test is easy to deliver and quick to return results, making it the obvious choice for roadside cannabis testing. It also tests for THC in the saliva rather than THC-COOH, meaning it’s better at determining if consumption was recent compared to the urine test. So you don’t have to worry about that joint from three months ago. The only problem is that the “cottonmouth” side effect of smoking cannabis has been known to cause false negatives. The tests are pretty good at catching other drugs though.

One new area where we might see some actual improvement is in the manufacture of a THC breathalyzer that can detect the substance’s presence on the breath. These devices can supposedly test whether a person has smoked cannabis in the last four hours.

California-based Hound Labs has been working on a breathalyzer for years and only recently announced that it’s nearly ready for the commercial market. Just last week the company told reporters that it expects to roll out the machines this spring. Drivers might start seeing these devices as early as the summer (although it’s unclear how many police departments are ready to buy them).

The Tolerance Problem

The biggest problem with all of these testing methods—aside from accuracy concerns—is the timing. Both blood and saliva tests are said to determine if a user has consumed cannabis within the last 24 hours. As any user well knows, the effects of cannabis generally last between 15 minutes and an hour for moderate-to-experienced users and up to three hours for newbies.

What’s even more distressing is that it’s unclear how marijuana tolerance works. For those who use cannabis more than once a week, the common side effects associated with intoxication seem to decrease or disappear completely. It’s arguably the entire reason that the industry is geared toward producing higher and higher THC potency in plants and extracts; patients and heavy users have to search for stronger products when the cannabis stops being as effective.

According to a study published in the journal Brain Research back in 1993, users build a tolerance to THC because the repeated use of the endocannabinoid receptors causes them to become less effective at binding with cannabinoids.

In other words: The very people who will get the least accurate test results (in terms of timing) are the same ones who may not be experiencing the psychotropic effects that are the core concern behind testing in the first place!

The Training Problem

In all of this talk about testing for THC with the most sophisticated scientific means available, we’ve forgotten to bring up the best highness assessment tool available to police: A trained eye.

While tests may deliver false negatives or false positives, a well-trained officer who knows what they are looking for will often be much better at recognizing the level of intoxication at which a driver is or isn’t operating.

According to the jury instruction on cases of driving under the influence of cannabis or other drugs (not alcohol), “The state must prove to your satisfaction beyond a reasonable doubt … the defendant was under the influence of drugs to such a degree that the defendant was incapable of safely driving a vehicle.”

For an officer to determine if a driver is capable of safely operating a vehicle, methods like field sobriety tests (stand on one leg and recite the alphabet backward) and plain old visual and olfactory detection are the best ways to spot a driver under the influence, and proper training is clearly the best avenue for keeping our streets safe.