Does marijuana cause more car crashes?

October 25, 2017

"How does Marijuana affect my ability to drive?" is the question we're asking today. While there have been studies done on this, there are a lot of mitigating factors and not many definitive answers. This is a timely discussion because in 2014, the National Highway Traffic Safety Administration found that the number of drivers with cannabis in their system has increased by 50% since 2007 (and yet the amount of drivers with alcohol in their system decreased by 33%).


What will happen to your ability to drive if you get behind the wheel when you're stoned? Well, one of the most cited research articles on this topic found that,


"Both cannabis and alcohol were associated with increases in speed and lateral position variability, high dose cannabis was associated with decreased mean speed, increased mean and variability in headways, and longer reaction time, while in contrast alcohol was associated with a slight increase in mean speed."


If you'd like to see this in action, CNN did an interesting segment in 2013 that had medical marijuana patients go through a driving course while under the effects of cannabis. Their findings are similar to the conclusion reached in the above research article. 



This video introduces another variable, which is that regular cannabis users will have an increased tolerance to cannabis. This could affect the level of impairment that they have when driving under the influence. However, despite everything that we've said so far, every research study that we've come across has not developed a link between driving while stoned and an increased risk of causing a car crash. This is even echoed in CDC documentation on driving an marijuana. The CDC attributes this lack of correlation to the difficulty (and lack of) of testing the presence of THC in drivers at the scene of a car crash – but that might not be the only reason.


NORML, a pro-cannabis publication, compiled a list of studies that observed the interaction between cannabis and driving ability and found that almost all of them failed to find statistically significant correlation between drivers with cannabis in their system and car crashes. 


This brings us to an important question, how do police officers determine the presence of THC in a drivers system when they're pulled over? Well, there are legal limits to the amount of THC that can be in your system while you're operating a motor vehicle and these limits vary state to state. In Colorado, the legal limit is 5 nanograms of THC. But, despite the existence of a legal limit, that is not the criteria a police officer will use to determine your level of intoxication. Instead, a police officer will base an arrest on your "observed impairment."


This is because there is no cannabis equivalent to the Breathalyzer. Such an advent would change the game of enforcing laws regarding cannabis and driving, but it's incredibly difficult due to how cannabis metabolizes. If you want more on that topic, here is a great article on the issue by Scientific American.


One of the biggest qualifiers for a police officer to look for in "observed impairment" is the amount of weaving a car is doing between lanes, which does seem to increase in drivers under the effects of cannabis. 



As you can see, there is a fair amount of data on the impact of marijuana on driving ability, but there aren't too many conclusions, which is frustrating to both governing agencies and to readers like you. While we can't conclusively state how impaired you will be if you ingest cannabis before driving, it does seem to alter your behavior behind the wheel.


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