Marijuana Users Tend to Overestimate Their ImpairmentKatarzyna Białasiewicz/123RF

Colorado statistics show that marijuana-related traffic deaths have increased by 62% since the legalization of marijuana in 2013.

Simple tests can detect alcohol impairment, but determining whether a driver is impaired by marijuana is much more complex. Still, law enforcement officials are working to assess the effect of marijuana use on traffic safety.

Each year the Rocky Mountain High Intensity Drug Traffic Area, a division of the White House Office of National Drug Control Policy, reports how the legalization of marijuana in Colorado has affected impaired driving, youth and adult marijuana use, and marijuana-related hospital admissions.

Among the findings of the 2016 Rocky Mountain High Intensity Drug Traffic Area report are:

  • From 2013 to 2015, average yearly marijuana-related traffic fatalities increased 48 percent compared with the average for the 2010-2012 period.
  • Marijuana-related traffic deaths have increased by 62% since the legalization of marijuana in 2013.
  • In 2009, the number of traffic deaths involving drivers who tested positive for marijuana was just 10% of all traffic deaths. In 2015 that percentage doubled to 21%.
  • 30 percent of all drivers who tested positive for marijuana in 2015 also tested positive for alcohol, and another 13 percent of marijuana-positive drivers had other drugs in their system as well.

Marijuana, Alcohol and Driving Impairment

Testing positive for marijuana use does not necessarily mean that a driver is under the influence of the drug at the time of a car accident. Maximum blood levels of tetrahydrocannabinol (THC) tend to show up before the driver becomes impaired, and as THC levels drop, impairment increases. Blood tests can detect traces of THC a day after the drug is consumed. Even though experts believe that the highest level of driving impairment associated with marijuana passes in a little more than three hours, there is no science linking impairment to a specific level of THC in the blood.

The body processes alcohol and marijuana very differently. Because of this, blood tests for alcohol can accurately measure the degree of impairment, but this is not true for marijuana.

While alcohol impairment increases proportionally with the amount consumed, marijuana’s effects vary broadly according to the individual. Factors such as body weight, previous use, and mode of delivery all determine how impaired someone may actually be.

The worst-case scenario is driving under the combined effects of alcohol and marijuana. Drivers under the influence of alcohol tend to underestimate how impaired they are, while marijuana users tend to overestimate their impairment and try to compensate for it by decreasing their speed and increasing their following distance. Alcohol also tends to increase a driver’s willingness to take risks, such as traveling at higher speeds or following other vehicles more closely. As a result, driving under the combined effects of alcohol and marijuana increases the risk of crashes and even intensifies the likelihood of psychotic symptoms, according to the Alcohol & Drug Abuse Institute at the University of Washington.

Legal History of Marijuana in Colorado

In November 2000, Colorado voters passed Amendment 20 to the state’s constitution, which legalized limited amounts of medical marijuana for patients and their primary caregivers. In 2010, the Colorado Medical Marijuana Code legalized marijuana dispensaries at the state and federal level. On January 1, 2014, Colorado passed Amendment 64, becoming the first state to allow the sale of recreational marijuana.

Embed this infographic:
Embed this image: