DUI Pot: Bad News & Good


Since the moment that the prospect of marijuana legalization became a reality, the race has

been on to invent a simple device to identify drivers impaired by marijuana, as breathalyzers

are used to identify drivers under the influence of alcohol. Such a device would provide an

acceptably objective measure with which a police officer can make an arrest likely to stand up

in court, supporting his determination about the driver’s state.


The bad news is that we’re unlikely to see one. The good news is that soon we won’t need one.

Breathalyzers don’t detect impairment. They measure the concentration of ethanol (the

psychoactive ingredient in alcoholic beverages) in a driver’s breath, which correlates directly to

blood alcohol concentration (BAC). If your BAC is .08% or higher, current law presumes you to

be under the influence and thus subject to arrest. The fairness of that presumption lies in

decades of testing and accident data indicating, according to the National Highway Traffic

Safety Administration, that “the vast majority of drivers, even experienced drivers, are

significantly impaired at .08 BAC in critical driving tasks such as braking, steering, lane changing,

judgment, and divided attention.”


In the case of marijuana, knowing—however accurately—the level of THC (the psychoactive

ingredient in marijuana) in a driver’s breath or body fluids provides no valid basis for presuming

impairment. No body of evidence shows that X mg of THC impairs “the vast majority of

drivers,” and one is unlikely to develop as impairment due to marijuana is a function of not only

the amount of THC consumed, but also the driver’s level of tolerance. Such levels vary wildly

among marijuana consumers, from the untutored novice who is rendered catatonic by a 15 mg

brownie to the seasoned stoner whom it would barely faze.


Measuring actual impairment—as compared to presuming impairment—is not impossible, but

no methodologies have earned widespread acceptance. In the 1970s, General Motors

developed a dashboard-mounted “critical tracking test.” Before the car would go, the driver

was required to keep a wandering needle on a gauge within an acceptable range by turning the

wheel. UMass Dartmouth professor Michel Milburn recently developed a cellphone app to

determine impairment by subjecting users to various physical and mental acuity tests.

The good news is that the threat to public safety presented by driving under the influence will

fade away as self-driving vehicles become ubiquitous over the next decade or two. Cars may

break down, but they don’t get high.


Until then, I have a simple suggestion. Let’s take some of the new marijuana revenue and equip all

police cruisers with video cameras. Let the officer record the operation of the car that drew his

attention and the driver’s response upon being pulled over. If the officer suspects the driver is

impaired, subject him to the standard roadside assessment/field sobriety test under the

watchful gaze of the camera. If the driver challenges his arrest, show the video to the judge or

jury, and let them decide.


If a driver is unable to recite the alphabet, walk a straight line or put virtual square pegs into

virtual square holes, she shouldn’t be behind the wheel, whether her impairment was caused

by alcohol, marijuana, prescription drugs or fatigue.


Facing the Boston Police Strike 100 years ago this year, Governor Calvin Coolidge famously

declared “There is no right to strike against the public safety, anywhere, anytime,” words that

propelled him to the vice presidency. It remains to be established that legal marijuana presents

any greater threat to public safety than illegal marijuana (and nothing like a police strike), but

with video evidence, we avoid any gamble with the public safety.