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Policy and Practice from the Front Lines of Marijuana's Political Economy

Marijuana Legalization–Part 3, Drugged Driving

December 12th, 2013

Using Marijuana While Driving

The Oregon legalization bill tackles getting high while driving, but leaves DUII laws unchanged.  If you light up (or its equivalent) in the car, you would be subject to the following, new proposed penalty:

Section 73.  Use of marijuana while driving; penalty. (1) A person commits the offense of use of marijuana while driving if the person uses any marijuana while driving a motor vehicle upon a highway. (2) The offense described in this section, use of marijuana while driving, is a Class B traffic violation.

Because this section does not alter DUII laws in any way, it serves as an additional punishment for those who are caught using marijuana while *safely* operating a motor vehicle.  The violation is similar to speeding (more than 10 MPH over posted limit, but less than 20 MPH), following to closely, etc. and results in a $300 ticket.  As a cogent reader can surmise, a cop who pulls you over for smoking marijuana while driving has probable cause to assess your ability to drive via field sobriety tests and/or breath/blood tests.

This legalization bill does not set THC blood concentration limits (as Washington did) and makes no alterations to the DUII laws–it seems that law enforcement personnel and future recreational users could use some clarification on marijuana’s effect on driving and the current tools available on DUII enforcement.  Others simply want to know:  does marijuana legalization require alterations to current DUII laws?

Stoned Drivers

First, lets address what the research has found on individuals’ driving behavior while under the influence of marijuana (and only marijuana).  It’s really important to emphasize upfront that the trifecta of marijuana, alcohol, and driving do not mix well (Robbe 1998; Ronen et al. 2010)–even in small amounts, the combination of alcohol and marijuana dangerously impairs drivers.  There is consensus within scholarly research on several important issues:  (1) marijuana does impair driving, (2) marijuana users compensate for that impairment by driving slow (well, not that slow, but 5-10 mph slower than their uninhibited selves), and (3) alcohol intoxication is far more dangerous in commonly used amounts of each respective drug.  Research on stoned drivers finds that–when results are statistically significant (they are not usually with low doses of marijuana)–they have slower reaction times and more difficulty maintaining their lane of travel than non-inebriated, non-distracted drivers.  As could be reasonably expected, this is accentuated by higher doses.  In all real world driving and marijuana consumption studies, stoned drivers were able to safely operate their vehicles and performed better than their legally drunk (> 0.08 BAC) counterparts.  Robbe (1998) notes at the conclusion of his double-blind comparative study of marijuana, alcohol, and marijuana+alcohol intoxicated drivers in the Netherlands that the “evidence from the present and previous studies strongly suggests that alcohol encourages risky driving whereas THC encourages greater caution, at least in experiments.  Another way THC seems to differ qualitatively from many other drugs is that the former’s users seem better able to compensate for its adverse effects while driving under the influence”.  This finding is echoed in much of the literature on stoned drivers (real world and simulated driving).

Dosing Issues and Cannabinoid Interactions

Real World Dosing

Dosing in these studies, however, is an issue; the most empirically valid research protocols control for body weight and administer real marijuana (much of it grown at the National Institute on Drug Abuse pot farm at the University of Mississippi) in doses based on micrograms of active THC per kilogram of body weight.  In the highest administered THC dose studies with real world driving assessments (300 micrograms THC / kg body weight) (Robbe 1998), stoned drivers performed about as well as drivers who had blood alcohol content (BAC) levels of 0.04%–affected, but well below the legal limit.

How does this dosing regime stand up to real world conditions?  Let’s take a 170 pound person (77.11 kg) using high quality marijuana with approximately 15% THC content as an example; how much real marijuana would it take to achieve similar levels of inebriation (300 micrograms / kg body weight) studied by scholars?

300 micrograms THC * 77.11 kg = 23,133 micrograms = .023 grams THC

.023 grams / .15 grams (i.e. 15% THC per 1 gram marijuana) =

0.153 grams of 15% THC marijuana

How does this stack up to the consumption patterns of regular marijuana users?  My research suggests that regular users of marijuana consume about .46 grams per day, though the concentration of THC and the amount per “use” is unknown.  If our fictional 170 lb user were to consume the Oregon average (.46 grams) of 15% THC marijuana in one sitting, he/she will receive a dose of 900 micrograms of THC per KG body weight–or exactly 300% of the most commonly used dose used in research on stoned drivers.  How do drivers perform at that level of intoxication?  No one has systematically studied this yet and it seems very important to conduct research on the effect of real-world doses on real-world driving performance.  Is it safe to assume that all marijuana users consume their daily allotment in one sitting?  Probably not, though such an occurrence cannot be discounted and also warrants further research.

Cannabinoid Interactions

The most glaring issue with all studies of marijuana dosing is derived from our nascent understanding of the interactions amongst cannabinoids when ingested.  Unlike alcohol, every individual marijuana plant has a unique effect on its users because of the variability in  their chemical composition (particularly cannabinoids and terpenes); in this regard, there is no realistic possibility of perfect standardization across marijuana studies or perfect extrapolation of clinical studies to real world situations.  Certain cannabinoids are known to interact with each other, with the two most important being THC and CBD (which is now viewed by many researchers as more medically beneficial than THC).  CBD is not psychoactive and is not likely to affect psychomotor abilities (that aspect has only been studied in mice so far).  Additionally, CBD inhibits the psychoactive effect of THC (i.e. the anti-marijuana drug is present in marijuana!), which means that varieties of the plant with higher CBD to THC concentrations will affect end-users’ driving ability less than their low CBD to THC counterparts.  Even with less or no impairment, individuals would have higher concentrations of active THC in their blood after consuming CBD-rich marijuana products (which calls into question the validity of THC ng / ml tests).  This means that measurements of THC are problematic (in plants or human bodies) without accounting for the antagonistic effects of other cannabinoids or terpenes.  Again, this is another area that requires significant scientific advances before any standard should be adopted.

THC in the Blood vs. Standardized Field Sobriety Tests (SFSTs)

5 nanograms of active THC in 1 milliliter of blood (5 ng / ml) is the marijuana impairment limit adopted by Washington.  This level appears to be based on Ramaekers et al. (2006) study on non-driving psychomotor performance exams and active THC concentration tests (blood), the authors recommended that 2 ng-5 ng THC / 1 ml of blood be established as the “lower and upper range of a THC limit for impairment” (114).  Unfortunately, the study is problematic in many critical ways.  First, in terms of research design, the authors themselves conclude that “the present study will be criticized for the face validity of the performance tasks and their ability to reflect driver impairment or crash risk” (120).  In other words, the tests used to assess impairment do not necessarily relate to real world driving performance.  That’s problematic when constructing a legal standard, though not surprising–the study was funded by the German Federal Police Academy and the German Society against Alcohol Drugs and Driving (Ramaekers et al. 2006: 121).  Second, other researchers flatly reject the notion that THC concentrations in the blood are related to impairment; instead, they suggest that high THC concentrations in the blood simply indicates very recent use (Robbe 1998; Papafotiou 2005a, 2005b; Karschner et al. 2009).  Ramaekers et al.(2006) study has been the only one to date that has recommended a standardized threshold for active THC in the bloodstream.

This point is driven home graphically in Papafotiou et al.’s work (2005a; 2005b) on Standardized Field Sobriety Tests (SFSTs) and marijuana intoxication (though dosing differs from gold-standard studies).  Papafotiou et al. provide mean THC ng / ml results over time for their 40 research participants (Fig.1, 2005a: 109), who ingested a placebo, low dose (0.813 grams@ 1.74% THC), or high dose (1.776 grams of 2.93% THC) of marijuana.


As the figure illustrates, marijuana smokers (an important, additional caveat) experience huge spikes in their THC/blood concentrations immediately after ingesting the drug (55-70 ng / ml, dose dependent).  Within 20 minutes, that concentration is reduced to 12.8-13.8 ng /ml, and after 100 minutes, all participants were below the legal limit established in Washington (3.18 & 3.72, respectively). How could this be?  The answer is remarkably simple.  Active THC in the bloodstream has not made it to receptor sites in the body yet and, therefore, have no effect on a person.  As active THC in the bloodstream is absorbed by the body (and declining in concentration), intoxication increases.  For this reason, Robbe (1998) notes this important finding from his study:

“Inter-subject correlations between plasma concentrations of marijuana and driving performance after every dose were essentially nil, partly due to the peculiar kinetics of THC.  It enters the brain relatively rapidly, although with a perceptible delay relative to plasma concentrations.  Once there, it remains even at a time when plasma concentrations approach or reach zero.  As a result, performance may still be impaired at the time that plasma concentrations of the drug are near the detection limit.  Therefore, an important practical implication of the study is that it is not possible to conclude anything about a driver’s impairment on the basis of his/her plasma concentrations of THC and THC-COOH determined in a single sample (S77)”.

In addition to measuring THC to blood concentrations, Papafotiou et al. (2005a; 2005b) also subjected their study participants to separate driving performance tests (at three time points) and three SFSTs at two time points (horizontal and vertical gaze nystagmus, walk and turn, and one leg stand tests).  At time point one, (30 minutes after smoking; all participants had more than twice Washington’s legal limit of THC in their system), all participants were “not significantly impaired in comparison to the placebo condition,” though no SFSTs were conducted (2005b: 175).  At time point two (80 minutes after smoking; participants’ average THC concentrations were below Washington’s legal limit), “driving was significantly impaired, as indicated by the ‘straddled the solid line’ variable..and the ‘straddled barrier line variable'”.  In other words, most stoned participants would pass Washington’s blood test standard, had trouble maintaining their lane of travel, and failed their SFSTs–however, that was the extent of their impairment.

Here’s the really important piece that may explain why Washington adopted a scientifically invalid standard.  Comparing the results of the SFST battery and driving impairment standards at 80 minutes after smoking resulted in the following findings:  “of the participants who were [scored as] impaired on the driving task, 88.5% were correctly identified as impaired but only 38.5% of participants who were not impaired on the driving task were correctly identified as not impaired” (176).  This means that stoned drivers who could not maintain their lane of travel were correctly identified as stoned 88.5% of the time, BUT 61.5% of non-impaired drivers failed the SFSTs and were accused of stoned driving when they really were not–even though all drivers were likely to pass the Washington THC / blood standard.  Even worse, a remarkable 0% of non-impaired drivers were identified 105 minutes after smoking.  Others have demonstrated the lack of face validity and scientifically tenuous nature of SFSTs (Rubenzer 2008); this finding corroborates that criticism in a demonstrative manner.  Field sobriety tests appear valid as a predictor of one form of marijuana impairment (inability to maintain lane), but are dangerously ineffective–0%!–at protecting citizens from false/erroneous accusations of impairment.  Restated, the THC / blood standard is invalid as a measure of impairment, but has the potential to protect citizens from DUII arrest if they wait an hour after smoking to drive.


People who consume marijuana products containing the full range of active cannabinoids experience diminished psychomotor abilities, but, like alcohol, moderate doses do not impair their driving ability enough to consider them unsafe.  That said, we need further research on “real world” dosing and its effects on driving ability.  There is no reliable lab measurement or standard (i.e. blood tests) to quantify marijuana impairment, in part due to the multitude of active constituents in the drug and because common reference tools (i.e. THC ng/ml) are not correlated with impairment in any meaningful way.  SFSTs remain the most effective tool for law enforcement to assess driving impairment; paradoxically, the problematically high “false positive” findings of SFSTs in research settings suggests that drivers suspected of marijuana use may require a more efficacious mechanism to demonstrate their lack of impairment.  In short, current DUII laws are more than sufficient to deal with marijuana intoxication–we do not need new standards for this particular drug.

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