THC Driving Limits: A Shot in the Dark


In California, a new ballot initiative to legalize marijuana is being planned for 2016. In response to legalization, California may follow in the footsteps of Washington and Colorado by enacting a “per se” law to combat the supposed dangers of driving under the influence of marijuana (DUIM). A per se law would create a presumption of DUIM if the driver tests positive for certain amount of THC, a psychoactive component found in marijuana. While similar laws exist nationwide for driving under the influence of alcohol (DUI), marijuana is not the same drug, and a DUIM law that doles out punishment based on a single biological measure may create unjust results.

What is a “per se” law? Essentially, a per se law makes a certain act illegal without needing proof of any surrounding circumstances. A well-known example is the .08 blood alcohol concentration (BAC) limit for enforcing DUIs. In every state, if you are caught driving with a BAC level over .08, you are presumed to be intoxicated, and that fact alone is enough to support a criminal charge. In response to marijuana legalization, a new per se law has been enacted in Colorado and Washington – if you have 5 nanograms of THC per millimeter of whole blood, you are presumed to be too high to drive.1 Given the widespread use of whole blood THC concentration as a metric for DUIM, this type of testing will be the focus of this article.

The current statute criminalizing DUIM in California requires proof that a driver was unable to operate a vehicle with the same caution and safety of a sober person.2 This is unlike a per se law because there is no single piece of data that will, by default, support a criminal charge. Instead, juries can consider all the evidence they are given and have it guide their judgment on whether a driver was or was not able to safely drive. In a post-legalization California, the goal of DUIM law should be to prohibit dangerous driving. To meet this goal, a per se law needs to be able to have an accurate way to test for intoxication, and the level of intoxication must correlate with an increased risk of crashing. Many proponents point to DUI per se laws as a model of success, but alcohol and marijuana differ both in the danger they present and the ability to accurately test for impairment.

DUI per se laws have been built on a foundation of scientifically proven crash risks. Since DUI law criminalizes a condition that may cause harm, the chance of causing harm was vital in the creation of DUI per se laws. In 1964, a study took place in Grand Rapids Michigan involving 9,353 drivers involved in car crashes and 8,008 drivers who had driven at the same times and locations without being involved in a crash.3 The Grand Rapids Study found that drivers with a BAC of .08% were twice as likely to crash, those with a .10% were six times as likely, and those with a .15% were ten times more likely.4 These results were confirmed in the many studies that followed after. (See pg. 45.) This correlation between BAC and crash risk gave lawmakers the ability to design per se laws that would criminalize a certain amount of risk. (See pg. 39.) In all states, double the crash risk, or .08% BAC, incurs a criminal penalty.

On the DUIM side of things, states like Washington and Colorado have used a 5 nanogram THC limit, and some states have used limits of 2 nanograms and lower.5 However, unlike the development of DUI per se law, a DUIM per se law cannot be designed around the risk of accidents. First, a significant crash risk for marijuana has not been definitely established. Second, THC’s presence in the body does not track impairment accurately.

As discussed in my previous article, the National Highway Traffic Safety Administration (NHTSA) recently released the results of a study that did not find an increased crash risk for drivers who tested positive for THC. The NHTSA study is the first large-scale study of its kind conducted in the United States and it would have been the best indicator of a crash risk, had there been one. This result is not through some trick of statistics or a poorly done study, but is likely owed to the fact that high drivers can compensate for the effects of marijuana by driving more conservatively. In contrast, drunk drivers are unable to compensate for their intoxication, and the NHTSA study confirmed that drivers with a .08% BAC had four times the chance of crashing.

If some crash risk is later shown, or the effect of marijuana on driving is enough to justify a per se law, its application would still run the risk of indicting drivers who aren’t intoxicated. THC testing presents numerous issues that remove it from the accuracy and fairness associated with BAC testing. Alcohol quickly equalizes between the brain, blood, and other tissue, so the level of alcohol in the blood (BAC) is similar to the level of alcohol intoxication in the brain. (See pg. 140.) THC does not reach a uniform concentration throughout the body, so the level of THC found in the blood will not consistently match the level of THC in the brain.

Furthermore, THC tends to remain in the body far longer than the duration of its effects. The impairing effects of marijuana will normally wear off in approximately three to five hours after smoking. However, THC can still be detected for as long as eight to twelve hours in infrequent users, and heavy users have shown to still have detectable levels of THC for several days. Since 50 or to 100 nanograms of THC is the typical peak, marijuana users may be unfairly targeted, especially in states with zero tolerance policies or THC limits as low as 2 nanograms. (Pennsylvania and Ohio, respectively).6 It is entirely possible that marijuana users who smoke on a Friday night could be arrested for DUIM just by driving the next morning.

These values reflect plasma or blood serum THC levels. Divide these values by about half for whole blood levels. (Transform Drug Policy Foundation, How to Regulate Cannabis: A Practical Guide (2014), pg. 176.)

These values reflect plasma or blood serum THC levels. Divide these values by about half for whole blood levels.
(Transform Drug Policy Foundation, How to Regulate Cannabis: A Practical Guide, at 176 (2014).)

Since THC concentrations generally fall between .5 and 2 nanograms in three to four hours after smoking, (see pg. 176) using a higher nanogram limit will reduce the chance that drivers are punished based on a lingering amount of THC in their blood. While it is possible to set a THC limit just high enough to avoid charging those with a residual level of THC, there is still a risk of charging drivers who have used a negligible amount of marijuana but are tested around the peak of their blood THC concentration. While higher THC limits help focus on drivers who have smoked more recently, the true goal of testing should be to catch dangerously intoxicated drivers, regardless of when they smoked. Given that blood THC concentrations poorly reflect intoxication, testing fails to separate dangerously intoxicated drivers from those who simply use marijuana and drive.

Just because there isn’t a per se law doesn’t mean that DUIM is legal or that the current law isn’t working. DUIM is a crime in California and will continue to be after legalization. When investigating a possible instance of DUIM, police officers will take note of many different factors, like the subject’s driving, behavior, performance on field tests, blood tests, and possession of drugs and/or paraphernalia. This body of evidence would then be used by a prosecutor to substantiate the criminal charge.

Proponents of a per se law argue that a legal limit for THC would establish a clearer method of charging intoxicated drivers, as a test result is all it takes to prove that someone could not safely drive. While this will make it easier to charge marijuana users who drive, there would be no guarantee that officials are only charging those who were dangerously impaired. Ignoring this issue and relying on the courts to fix it would be to “let god sort them out.” Criminal indictments create significant hardship for defendants, regardless of whether they may be found not guilty later on. Furthermore, many defendants would be financially incapable of hiring an expert who could educate the jury on the inaccuracies of THC testing, leading to disparate outcomes in the judicial system based purely on wealth. A per se law runs the risk of arbitrarily causing hardship for marijuana users, acting more as a blanket prohibition than targeted deterrence.

While the current law may not be perfect, it still takes into account many different facts to establish whether a driver is dangerously impaired, cutting straight to the goal of DUIM enforcement. On the other hand, a per se law and the resulting tests would not accurately differentiate between those who pose a danger the public and those who merely use marijuana and drive. DUI per se laws were developed around a consensus of evidence that showed an increase in crash risk for BAC levels as little as .05%. (See pg. 45.) Because there is not a similar level of evidence tying blood THC concentration to accidents, lawmakers cannot criminalize THC levels based on validated safety risks. With no evidence of harm and no accurate way to test intoxication, enacting a DUIM per se limit would not be a shield against hazard, but more like a shot in the dark.

1. Colo. Rev. Stat. Ann. § 42-4-1301(6)(a)(IV); Wash. Rev. Code Ann. § 46.61.502(1)(b)
2. Cal. Veh. Code § 23152(e); CALCRIM 2110
3. Robert F. Borkenstein et al., The Role of the Drinking Driver in Traffic Accidents (1964).
4. Id.
5. Colo. Rev. Stat. Ann. § 42-4-1301(6)(a)(IV); Wash. Rev. Code Ann. § 46.61.502(1)(b); Ohio Rev. Code Ann. § 4511.19(A)(1)(j)(vii); 75 Pa. Cons. Stat. Ann. § 3802.
6. 75 Pa. Cons. Stat. Ann. § 3802; Ohio Rev. Code Ann. § 4511.19(A)(1)(j)(vii).



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