When it comes to driving, you’re probably familiar with the idea of a legal limit.

You measure the amount of alcohol  in someone’s breath or saliva at the side of the road, and if it’s  above a certain concentration, law enforcement can presume that that  person is too impaired to drive. And having a specific cutoff  number is a good thing – in theory, anyway.

It gives us a way to tell if  alcohol is being used in a way that doesn’t endanger the user or other people! And with legal marijuana becoming more common, you’d think similar sensible  safeguards would come into existence. But they haven't.

Some states in the US, like  Arizona or Rhode Island, have zero tolerance for driving  after consuming cannabis. Others, like Illinois, Nevada or Washington, take an approach like we have for alcohol. They’ve set a specific limit for THC — the psychoactive compound in weed — ranging from 1 to 5 nanograms  per milliliter of blood.

The trouble is, that doesn’t work. See, cannabis is just so weird  in how it affects the body, that we just don’t have a  good test for driving high. And the more we learn about cannabis, the more some researchers think  we may never have one at all. [INTRO] To understand why THC impairment  is so hard to test for, we first have to understand why testing for alcohol impairment is so easy.

We have decades of research  about how alcohol is metabolized, that is to say, how it makes  its way from the mouth, around the body, and eventually to the liver. Plus, we know how the concentration of alcohol changes at each of those points. This is important when it comes to testing, because it means measuring the level  of alcohol in the blood or breath. soon after drinking is a  reliable indicator of the levels in other parts of the body  that are much harder to get to. /And/ we know how alcohol affects our brains, and therefore how it affects  our behavior and reaction times, the kind of things that make driving  under the influence dangerous.

Once someone hits 0.03 grams of  alcohol per 100 milliliters of blood, they might be more confident, and by 0.09 grams they might not be able to  think so critically anymore. By 0.25 grams, some people lose the ability to control their movements. That strong base of evidence means most countries around the world feel confident  adopting a limit for blood alcohol around 0.05 grams of alcohol  per 100 milliliters of blood — right before they get drowsy or  their vision gets too blurry.

There’s a number, it can be reliably measured, and it roughly applies to everybody. But unfortunately cannabis is different. When we’re talking about marijuana, we’re talking about the molecule that is most likely to mess with your ability to drive: THC.

That’s as opposed to other molecules like CBD. Pretty much everything that makes alcohol easy to test for doesn’t work when it comes to THC. Starting with how it's metabolized.

It’s possible to detect THC in  blood plasma just seconds after the first puff, with the concentration  peaking around 8 minutes later. After that, THC just loves hanging out in fat, so it quickly makes its way to  any organ that’s really fatty — like lungs, the liver, and of course, the brain. That means, within literal minutes, the level of THC in the blood is  much lower than that in the brain.

And right now, the only way to check the level of blood THC is to have a professional draw blood. There are saliva tests, but  we’ll come back to them. And since most police officers  don’t have a phlebotomist riding shotgun, after an accident or  suspect DUI, this can take hours – meaning blood levels are likely to have gone down even if the person did have a  recent exposure to cannabis.

THC is then metabolized by the liver — a process that can take an  average of 12 to 60 hours depending how frequently the  person is exposed to cannabis. There’s also this other weird  phenomenon where blood THC levels peak a second time a few hours  later after exposure, as metabolized versions of the compound move back  into the blood from the liver. All that means taking single  snapshot of your body from a vial of blood simply isn’t going to tell  you how much of the mind-altering substance is in the place where  it can… well… alter the mind!

What’s more, unlike alcohol, THC levels vary depending on whether it’s  smoked or consumed orally. If it’s ingested, it can take hours  for the levels of blood THC to peak, and levels stay pretty low  overall compared to smoking. And ingesting THC produces  less of a high than smoking it.

Uh! Or… So I’m told… from.. Research papers.

Not only do THC blood levels vary depending on how someone takes it, but also how often they do so, because it accumulates in the body. Like, an occasional smoker  might only have a trace of THC in their blood after a few  hours, whereas someone who smokes every day can build up so much  THC in their fatty tissues that it keeps seeping out for up to a  month after their last exposure. So a habitual user might have a constant level of THC in their blood, even if their brain isn’t high So blood THC cannot tell  us specifically how recent someone's exposure was, how much they’ve had, or how impaired they might be.

There just isn’t a simple, single cutoff for THC like there is for alcohol,  and there really can’t be. We’ll talk about what that  means after this quick break. This SciShow video is supported by Hungry

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After all, that’s what’s really important here. Whether the person is putting  themselves or others in danger. Remember, the blood level is  just supposed to be a proxy, something we can objectively measure that stands in for what we actually want to know.

At least one researcher says  that heavy consumers can be cognitively impaired for up to 28  days after their last exposure. But one 2023 study found no correlation  between THC levels in either blood or saliva and how well  someone did on a driving simulator. And that study fit with a meta-analysis  from a year earlier, which concluded that, quote, “Blood and  oral fluid THC concentrations are relatively poor indicators of  cannabis/THC-induced impairment.” Still another simulated driving  study from 2020 found that some people were barely impaired  when over a certain limit, while others were highly impaired  when under that same limit.

In most of these studies,  researchers are measuring things like how much a person  weaves from lane to lane, or how close they are from the car in front. But we need some kind of measure that works on the road, not just in a lab. Because there’s a widespread belief that consuming cannabis makes you a better  driver, not a worse one.

In fact, /82%/ of regular cannabis users said they would drive under the influence,  according to one 2005 study. And most of them thought their  driving would be only mildly impaired, or even better, if they did. Well, at least according to a 2013 review, the whole “better under the influence”  myth doesn’t seem to be true.

Several studies in that  review linked driving under the influence of cannabis with fatal crashes, even when other factors like risky behavior or age were taken into account. And other lab-based studies  said that it’s more difficult for someone who’s high to divide  their attention between two tasks, or stick to a lane on a driving simulator. But maybe people think they’re  driving better under the influence of cannabis because they drive  more slowly, potentially because they know they’re high so  they’re trying to compensate.

Trouble is, they still drive worse than someone who isn’t under the influence! So in case you needed to hear me say it clearly: Don’t drive stoned. So, what do we do?

If we  can’t find that magic number, is there any way to reliably check for cannabis impairment in the name of public safety? One solution that’s been proposed  is to collect capillary blood with a needle that automatically and easily does a pinprick draw on someone’s shoulder. That could mean the police  could get a result sooner than with a regular blood  test, which would help overcome the problem of blood THC  levels going down so quickly.

Trouble is, these tests often underestimate the amount of THC floating around in the blood. And it doesn’t solve the other problems, like blood THC not always  reflecting time of exposure. And what about testing some  other fluid, like saliva?

Well, those tests have their issues too. They’re less sensitive and less specific — meaning they’re not as good at  picking up subtle differences in THC levels and therefore  whether someone is over the limit. Some studies have even said there’ll be a large number of false positive tests.

Alright, but there’s still the  ol’ fashioned impairment test. And I know what you’re thinking, what about the good ol’ fashioned impairment test? This could be something like  walking heel to toe along a line or standing on one leg while  listening to other instructions.

You might have seen them in movies, and they’re actually used in real life too. Except these tests aren’t great  either, for a couple of reasons. One is that there’s no universally agreed on task for measuring if someone is too impaired to drive.

Combining three tests, like in the standardized field sobriety test, is better, and ends up being around 86% accurate for alcohol, at a BAC of 0.05. But to get that kind of accuracy, you need to have trained  professionals on the scene. Even then, it’s still a subjective test.

In one double blind study, for example, DUI conclusions for cocaine or  marijuana matched toxicology results in only 44% of cases,  and 6 out of 16 people on a placebo were considered impaired  when they clearly weren’t. Plus the full test lasts around an hour, so you kind of end up with the  same problem as a blood test where some of the effects  may have worn off already. And you of course can’t use this kind of test in fatal crashes, or for forensics.

So, for now at least,  researchers are a little stumped. Maybe that’s not a satisfying answer, but it’s an uphill battle against  basic chemistry and physiology, and knowing what we don’t know  is the first step to a solution. A single legal cutoff probably  isn’t it, which leaves researchers still searching for  the test that actually works.

The one that will keep drivers, and those on and around our roads, safe. [♪OUTRO]