This is a guest post, originally published by Ed Wood of DUID Victim Voices.
THC (delta-9 tetrahydrocannabinol) is the primary psychoactive ingredient in marijuana that causes driving impairment. Legislators have long sought a per se level of THC in a driver’s blood that is equivalent to alcohol per se levels, .08 BAC in the United States and .05 BAC in most other countries.
“The Futile Search for the “Right THC per se Level” describes why there is no scientifically valid THC per se level. It is not because we need more studies. We have done the studies and we know that such a level cannot exist and why it cannot exist. Alcohol is the only impairing drug that has a good correlation between blood drug level and level of impairment.
There is no level of THC in blood above which everyone is impaired, and below which no one is impaired. But that fact has not prevented Washington, Montana and Illinois from setting an arbitrary 5-ng/mL THC per se level in whole blood. Other states are poised to follow their bad example.
The fundamental flaw with any THC per se limit is that impaired drivers below any arbitrary limit are unlikely to be convicted. The number of impaired drivers who test below 5-ng/mL is not trivial. Reports from Colorado, Washington, Pennsylvania and Sweden reveal that among THC-positive drivers arrested for impaired driving, between 50% and 90% test below 5-ng.
Some claim that a 5-ng limit will convict non-impaired drivers above 5-ng. While possible, that is unlikely because a police officer cannot even request a blood test unless a driver demonstrates evidence of impairment. To do otherwise is violation of the 4th amendment to the US Constitution which courts will not allow. Nevertheless, the claim is not without merit.
The above states do not report either improper exoneration of THC impaired drivers below 5-ng or conviction of non-impaired drivers above 5-ng because they do not measure, analyze and publish their DUI data as Colorado has done since 2017.
Not only are THC per se laws scientifically invalid, but we also now have evidence that in practice, a 5-ng level has the following flaws:
It does not work,
It is not necessary to achieve a proper conviction, and
It is being abandoned by prosecutors.
This evidence comes from Colorado, the only state to collect, analyze and publish DUI statistics that enable us to answer questions about the efficacy of a 5-ng THC law.
Colorado has a 5-ng THC whole blood permissible inference law. Unlike a per se law, it allows a court to infer that a driver was impaired if the 5-ng limit was exceeded, but only if other evidence supports that conclusion. That structure was put in place to ensure that non-impaired drivers above 5-ng would not be convicted of driving under the influence.
Colorado has not one, but two different impaired driving violations :
DWAI Driving While Ability Impaired, and
DUI Driving Under the Influence.
Colorado’s law defines DWAI as impairment to the slightest degree so the driver is less able than he ordinarily would be to drive safely. The more egregious DUI offense is defined as the substantial inability to drive safely. Colorado’s law applies only to DUI; there is no THC permissible inference for the lesser DWAI offense. Colorado’s inconsistent application of permissible inference allows us to compare conviction rates for THC-impaired driving with and without a whole blood 5-ng permissible inference level. Nowhere else in the world can this be done.
The tables below are from the Department of Public Safety , charged with the responsibility to publish DUI statistics since 2017. The data are from calendar year 2019, the last year for which data are available. Unpublished data from 2018 show similar results.
The top table below shows THC only cases, the bottom table shows alcohol only cases. Polydrug cases are not included here. Each table shows two offenses: DUI in the top two rows, DWAI in the bottom two rows. Each offense shows charges and convictions for drivers above and below the respective legal limits: 5 ng for THC and .08 gm/dL for alcohol.
Observe the following:
|DUI 5 ng+||221||155||70%|
|DUI < 5 ng||98||8||8%|
|DWAI 5 ng+||372||370||100%|
|DWAI <5 ng||56||52||93%|
|DUI BAC .08+||5,397||5,032||93%|
|DUI < BAC .08||91||24||26%|
|DWAI. BAC .08+||4,096||4,094||100%|
From the above, we reach the following conclusions:
Incidentally, the high prevalence of polydrug impaired driving makes drug per se laws even more problematic. Since drug effects are additive, a driver with a non-impairing dose of alcohol combined with a non-impairing dose of THC can be high impaired. Yet that driver could not be convicted under per se laws.
So, what should states do?
“If a driver’s blood or oral fluid contains a measurable level of one or more impairing substance other than alcohol, such fact gives rise to a permissible inference that the driver is impaired.”
Alcohol should be an exception because its per se level has been well established as being effective.
Further information on this topic is available on the Every Brain Matters webinar.