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.