CONCLUSION
In this book, I have compared our knowledge of the effects of alcohol and cannabis
in relation to driving and Canadian legislation to reduce driving casualties. The safety
benefits of the Breathalyzer for detecting alcohol impairment are substantial and
undeniable. A large body of credible research shows alcohol negatively affects performance
in a dose-response relationship and the BAC level from breath tests is an excellent measure
of performance deficits. As well, alcohol is eliminated from the body at a constant rate and
is water-soluble, which means there is a close relationship between alcohol in the blood
and performance. Although alcohol can cause long-term effects, such as those due to
hangovers, the Breathalyzer cannot detect such deficits.
For cannabis detection, the story is much different. Blood tests, considered the
most valid biological sample, are inconvenient, difficult to administer in a timely fashion and
have poor relationships between low levels of THC and performance deficits. Based on the
preponderance of research, a THC range between 2 and 5 ng/mL and a cut-off and above 5
ng/mL for impairment are very low compared to the alcohol cut-offs at .05%. As well, the
theory of 24-hour impairment is best described as a myth and does not provide an evidence-
based rationale for adopting low THC cut-off levels. The preponderance of research
evidence shows cannabis hangovers are not meaningfully related to performance deficits.
A very conservative lower per se impairment cut-off is 6 ng/mL and can be justified based
on one empirical observational study, and 8 ng/mL based on one laboratory study as
equivalent to .05% alcohol. These studies have not been replicated or validated. No credible
evidence of meaningful impairment below 5 ng/mL exists for smoked cannabis.
 
Some readers may be surprised at some of the findings in this book, yet many of
the conclusions I have drawn are not original. There is growing agreement that the
detection of THC in blood is associated with ORs between 1 and 2, a weak effect size.  
 
In fact, many of my findings were also independently found by special task forces. For
example, in Canada a Task Force (McLellan et al., 2016) aimed at studying the implications
of cannabis legalization arrived at conclusions below that are consistent with those of this
book:
 Cannabis-impaired driving is more complex to study than alcohol-impaired driving;
 Whereas evidence was gathered over many years to arrive at an established metric
for alcohol intoxication - Blood Alcohol Concentration (BAC) - these types of data
do not exist for cannabis; While scientists agree that THC impairs driving
performance, the level of THC in bodily fluids cannot be used to reliably indicate
the degree of impairment or crash risk;
 Other challenges exist, including the need to account for the rapid and sharp decline
of THC levels in the blood in the hours following consumption through smoking
(with edibles the decline is more gradual).
Furthermore, Peterson et al. (2019) concluded:
 Research studies have been unable to consistently correlate levels of marijuana
consumption, or THC in a person’s body, and levels of impairment. Thus some
researchers, and the National Highway Traffic Safety Administration, have observed
that using a measure of THC as evidence of a driver’s impairment is not supported
by scientific evidence to date.
178
 The body metabolizes marijuana differently from alcohol. The level of THC (the
psychoactive ingredient of marijuana) in the body drops quickly within an hour after
usage, yet traces of THC (non-psychoactive metabolites) can still be found in the
body weeks after usage of marijuana.
 The National Transportation Safety Board has advised that “about 2 alcoholic
drinks” within an hour will cause a 160-pound male to experience decline in visual
functions and in the ability to perform two tasks at the same time. Based on current
knowledge and enforcement capabilities, it is not possible to articulate a similarly
simple level or rate of marijuana consumption and a corresponding effect on driving
ability.
 Several factors complicate the effort to determine what, if any, impact marijuana
usage has on the likelihood of being involved in a crash. Chief among these factors
is the distinction between correlation (things that occur together) and causation
(one thing that causes another thing).
In this book, the applicability of biological drug tests to detect a meaningful safety
risk from cannabis use were reviewed. Each of the main types of biological drug tests—
blood, oral fluids and urine—were poor for detecting safety risks due to cannabis. As well,
although the safety risk for cannabis use can be meaningful, they are minor in comparison
to alcohol. These findings suggest that interventions to prevent cannabis-impaired driving
are warranted. Since the drug tests by themselves are not viable in terms of valid
conclusions, we are left with identifying some other criteria to identify safety risks related
to cannabis. Other types of behavioural tests are possible. Unfortunately, our present
knowledge of these tests also indicates they are not valid. The question of importance is,
what can be done that is reasonable? Certainly, legislation prohibiting drivers from using
cannabis while driving seems reasonable. As well, if drivers exhibit major behavioural
symptoms of performance deficits, intervention is warranted.
In terms of safety issues related to drivers, we know that distracted driving is
unsafe. The root causes of distracted driving are when attention is divided between driving
tasks and other activities. Smoking, eating and drinking non-alcoholic beverages while
driving increase safety risk. Yet, these are issues that could be easily addressed with
legislation. Manufacturers of cars are permitted to install cup holders and ash trays in
automobiles. Fast food take-out restaurants are permitted on major highways. These are
likely safety issues that appear to be endorsed in our society, and legislation could easily be
enacted to prohibit these encouragements, but political will to act is weak. Our priorities
need to shift from beliefs of what should be done to empirical research of what is unsafe
and where feasible, enacting equitable policies to address safety risks related to driving.
An issue described in this book is how we choose our priorities for improving safety.
I believe perceptions on how to address cannabis and driving stem largely from our
knowledge of alcohol and driving. Clearly, alcohol is a menace on the roads, and the
Breathalyzer has been useful as a deterrent in preventing fatalities, injuries and crashes.
Since alcohol has been legal in most countries but cannabis is not, many people may think
that cannabis must be worse than alcohol, yet this is not true. As shown in this book, many
of the lessons we have found from alcohol and driving cannot be applied to cannabis and
driving

Results: Marital separation or divorce was associated with an increased risk of a serious accident (all serious accidents: hazard ratio 2.9, 95% confidence interval = 1.7-5.0; at-fault accidents: 4.4, 2.3-8.3). The impact of separation and divorce did not differ according to alcohol consumption levels. Other life events associated with increased risk of serious accident were a child leaving home (all accidents: 1.2, 0.97-1.6; at-fault accidents: 1.5, 1.1-2.1), an important purchase (all accidents: 1.4, 1.1-1.7; at-fault accidents: 1.6, 1.2-2.1), and hospitalization of the partner (all accidents: 1.4, 1.1-2.0).

These findings suggest that interventions to prevent cannabis-impaired driving
are warranted.
[...]
Certainly, legislation prohibiting drivers from using
cannabis while driving seems reasonable.