How high is too high to drive? The challenges facing legal marijuana limits

By | January 6, 2019

How high is too high to drive? Weed was found in 40% of car victims in 2016 – but it is a LOT more complicated to test for than alcohol

  • More people who died in DUI accidents in 2016 had marijuana in their systems than had alcohol 
  • As marijuana legalization sweeps the nation, public health officials and policy experts are struggling with how to set legal driving limits 
  • There are breathalyzers, blood and urine tests as well as field tests for impairment 
  • But two experts explain why none of these are a perfect way to get drivers who are too high off the road  

Marijuana is now legal for medical use in the majority of US states, for recreational use in 10 and counting and while that is bringing individual health and business benefits to those areas, it introduces a new public health challenge: high driving.

Marijuana was found in the systems of 40 percent of people who died in car accidents while driving under the influence of a substance in 2016. 

Yet some studies suggest stoned drivers are ‘safer’ drivers while others suggest their reflexes are slower. 

And determining when someone is too high to drive is a problem that has thus evaded the chemists, psychiatrists, law enforcement agents and public policy makers. 

Two scientists explained to Daily Mail Online why the most scientific tests may not be the best way to determine how high is too high and the unique chemistry that makes marijuana a drug-screening enigma. 

Marijuana was involved with 40 percent of DUI deaths in 2016, but determining how much someone has to smoke to be too high to drive presents serious scientific and policy challenges

Marijuana was involved with 40 percent of DUI deaths in 2016, but determining how much someone has to smoke to be too high to drive presents serious scientific and policy challenges

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There are a number of biochemical reasons that establishing a legal limit for marijuana is so difficult. 

Some are unique to the drug, but one is true of just about any substance: tolerance.  

Depending on how often and in what doses someone uses any substance, the amount it takes to for the drink or drug to have an effect varies drastically.  

To begin with a more familiar example, ‘there are lots of people who drink every day and can walk a perfectly straight line at .08 [percent blood alcohol, the legal limit in the US),’ says Dr Richard Clark, director of the division of medical toxicology at the University of California, San Diego.

‘But then there’s college students who are practically comatose at .08.’ 

The same is true for marijuana. The more frequently your endocannabinoid receptors are exposed to THC – the psychoactive ingredient in cannabis – the less sensitive they become to the effects. 

So, people with medical marijuana prescriptions, for example, might smoke weed every day, seem totally unaffected and breezily pass a field sobriety test – which involves tests for coordination and balance, like walking the line, as well as some for memory and attention – even though they’ve recently ingested lots of the drug. 

Beyond that, marijuana moves through the body in very different way from alcohol.  

With a healthy liver, alcohol moves through your bloodstream at a rate of a bout one drink per hour, but each drink within an hour tacks on more time. Breathalyzers can continue to detect alcohol in your blood for about 24 hours after drinking. 

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Its effects on the brain dissipate relatively quickly. 

Marijuana – more specifically, its psychoactive component – leaves the blood very quickly, but it lingers in the fat and brain, meaning its cognitive effects do, too. 

According to Dr Clark, marijuana may even move from these tissues back into the blood days later in ‘chronic’ smokers. 

And just to add an extra level of difficulty, the THC in increasingly popular edibles gets converted quickly to anther compound in the stomach, so a THC test might not even detect it, even when a high was in full effect.  

 ‘So we’re sort of stuck,’ says Dr Clark. 


Two of the first states to legalize medical and recreational marijuana – California and Colorado – took two very different approaches to testing drivers for impairment. 

California did not establish a legal limit for THC in the blood. Instead, the state decided that someone is automatically too high to drive if they fail a filed sobriety test. 

In Colorado, a legal limit was set to five nanograms of THC in the blood, though police officers can also use their discretion to determine that someone is not fit to drive. 

Blood and urine tests are available, but sometimes a long time passes between when someone is pulled over and when the test can be administered. 

There are two recently developed breathalyzers for THC – one from Hound Labs and another from Cannabix – and several other tests in development, but these face the same challenges of disparity between blood level and actual high. 

‘So you’ve got a situation where you’ve got people who [have] low [levels of THC on their breath] and too impaired to drive or low and not impaired,’ says Dr Thomas Marcotte, co-director of UCSD’s Center for Medicinal Cannabis Research. 

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Dr Marcotte and his team are undertaking a massive study to determine exactly how to test for high driving. 

They are using a driving simulator to analyze things like how a high driver handles turning left through traffic, their perception of time, reflexes and decisiveness when they meet a yellow light. 

Previous research tells us already that high drivers are ‘less efficient at dealing with multi-tasking – which none of us should do but we all do while driving,’ Dr Marcotte he says. 

They are also working on an iPad field sobriety test. 

It’s too soon to say what will come of Dr Marcotte’s field tests, but as we stand now, no method is perfect.  

‘Field sobriety testing introduces subjectivity into something you’d really like to be subjective,’ says Dr Hall.  

But, he still prefers this to the Colorado way. 

‘I like the way California does things … my opinion is that the people you don’t want on the road are the people that are impaired, and blood levels don’t predict that as well as certain field tests,’ he says. 

And there’s scientific evidence on both sides: ‘There are suggestions that some people who drive after using marijuana drive more carefully because they know they are high, but reflexes are impaired under marijuana. They’re higher than that if you’re sober, but lower than that if you’ve drunk alcohol,’ explains Dr Hall. 

‘So what does it all mean – except it’s better to drive completely sober.’      

Health | Mail Online