Attitudes toward driving under the influence of cannabis vary greatly. Some individuals claim that driving under the influence is extremely dangerous; whereas others suggest that cannabis is a fairly innocuous substance that does not increase your risk of collisions. Research indicates that the true answer is somewhere between these two extremes. Comparisons between studies of alcohol intoxication and collision risk versus studies of cannabis intoxication and collision risk show that alcohol has more detrimental effects on driving than cannabis has on driving. Conversely, some research has shown that drivers under the influence of high levels of cannabis attempt to compensate for their condition by driving slowly and cautiously. However, while compensation can help reduce the risks of collision, these risks are not fully eliminated.
A recent review of the evidence showed that cannabis doubled the likelihood of a collision, which would be similar to the likelihood of collision from having a blood alcohol content (BAC) of .05%, the legal alcohol limit in most Canadian provinces. For comparison, heavy alcohol intoxication to a BAC of .20% alcohol is related to over an 80 fold increased likelihood of a collision. Cannabis affects your attention span making it harder to concentrate on driving and makes it difficult to shift attention to deal with changes in the environment. Evidence suggests cannabis may also impair tracking ability (i.e., harder to follow your lane), and sense of time and distance. The duration of impairment from cannabis lasts on average about 90 minutes, but it is best not to drive within 4 hours after smoking cannabis.
It is a criminal offense to drive under the influence of cannabis, however, nearly half a million Canadians admitted to driving within two hours of using marijuana or hashish in 2006 and youth report driving after cannabis more than after alcohol (40% vs. 20%). If a police officer suspects that you are driving while under the influence of cannabis, you can be sent to the police station for examination of impairment by a drug recognition expert, and possibly be asked to provide a biological specimen, such as urine or blood for analysis. This process is time consuming and cumbersome for the police. Some countries have dealt with the issue by implementing roadside saliva testing – a test that can detect the use of cannabis over the past few days. For example, Australia has a zero tolerance policy for driving under the influence of cannabis and uses roadside saliva tests to assess for the presence of THC. However, saliva and urine tests are not as effective at detecting impairment compared with the Breathalyzer test used for alcohol, where increased blood alcohol concentration corresponds closely with safety risk. That is, a person who tests positive for cannabis on a saliva or urine test may not be under the influence at the time of the test and therefore their driving may not be impaired. Another more accurate option for detecting cannabis impairment is the use of blood tests, currently being used in Germany, however these tests are also more intrusive and difficult to implement at roadside.
Driving under the influence of cannabis is a serious public health concern and with the recent legalization of cannabis in Washington State and Colorado and the continued movement towards decriminalization of cannabis in BC, cannabis related driving policies will be an important component of any regulatory system.
Authors: Scott MacDonald, Assistant Director, CARBC, and Chantele Joordens, Research Associate, CARBC
Matters of Substance 
Thanks for bringing some attention to this matter. Very few people give much consideration to this issue, and when they do, they usually rationalize it by saying that it’s not as bad as alcohol. Thanks for showing that while that may be true, it still poses significant danger to society, and is something that does need to be considered.