I read an article in the Daily Hampshire Gazette concerning Cannabis Impairment and wanted to check around on the statistics involved. Does Cannabis impair driving? As an experienced onlooker I have not seen this to be a problem. Most people using cannmarijuana plantabis
are much more responsible when it comes to driving than their counterparts using alcohol. Most people that are “high” tend to not want to drive at all.

As it goes Cannabis does cause impairment, however studies have shown that people tend to overestimate this, and that they compensate for it with added caution. Alcohol tends to do just the opposite, people perceive their impairment to be less that what it actually is and often become overconfident, aggressive, and careless. [Robbe and O’Hanlon. 1993; Robbe. 1995]

To find out how cannabis use affects crash risk overall, in 2015 the U.S. government completed the largest case controlled study to date regarding DUI of cannabis and crash risk [Compton and Berning. 2015]. It involved over 9,000 cases and controls spanning a 20-month period. The researchers found that cannabis use while driving is not associated with increased crash risk once adjusted for confounding variables such as age, race, gender, and the presence of other drugs, including alcohol:

“This analysis shows that the significant increased risk of crash involvement associated marijuana-leaf red white and bluewith THC and illegal drugs shown in Table 3 is not found after adjusting for these demographic variables.”

Furthermore, they found that cannabis did not add to the crash risk for drivers under the influence of alcohol:

“As was described above, there was no difference in crash risk for marijuana (THC)-positive drivers who were also positive for alcohol than for marijuana (THC)-positive drivers with no alcohol, beyond the risk attributable to alcohol.”

They also found that alcohol greatly increased crash risk:

“at moderate alcohol levels (0.05 BrAC) risk increases to double that of sober drivers, and at a higher level (0.10 BrAC) the risk increases to five and a half times. At a BrAC of 0.15, the risk is 12 times, and by BrACs of 0.20+ the risk is over 23 times higher.”

These results agree with an extensive 2013 review of 66 studies regarding crash risk and drug use which found that cannabis was associated with minor, but not statistically significant increased odds of injury or fatal accident. [Elvik R. 2013]

It is clear that DUI on alcohol is much more dangerous than DUI of cannabis. With that being said, at some point a person could potentially be high enough to significantly increase crash risk, being roughly the equivalent of 0.08% BAC, and DUI laws should reflect that, be based on actual impairment, not unscientific “per se” limits. It takes very large amounts of cannabis to reach this level vs just a few drinks of alcohol. It is very rare for anyone this high to want to actually drive a car, whereas it is commonplace for someone legally drunk to attempt to drive, even at impairment levels far exceeding that of 0.08%, which are not even obtainable with cannabis.

Colorado legalized recreational cannabis in Dec 2012 (Jan 2014 for retail sales) and have not experienced a large surge in fatal traffic accidents:

2012: 474 (Population: 5.19 million, 0.0091%)
2013: 481 (Population: 5.27 million, 0.0091%)
2014: 488 (Population: 5.36 million, 0.0091%)
[SOURCE: Colorado DOT & “As Reported” to NHTSA by FARS]

Recreational cannabis use for adults was legalized in Washington state in Dec of 2012. Retail sales began in July, 2014. They also did not experience a large surge in fatal traffic accidents:

2011: 421 (Population: 6.82 million, 0.0062%)
2012: 403 (Population: 6.90 million, 0.0058%)
2013: 401 (Population: 6.97 million, 0.0057%)
2014: 429 (Population: 7.06 million, 0.0060%) (Preliminary data)
[SOURCE: Washington State Department of Transportation – 2012, 2013 Annual Collision Summary]

There are many factors that affect traffic fatality rates. Wyoming saw a large increase in traffic fatalities yet they did not legalize cannabis:

2011: 135 (Population: 567,631, 0.00238%)
2012: 123 (Population: 576,893, 0.00213%)
2013: 87 (Population: 583,223, 0.00149%)
2014: 150 (Population: 584,153, 0.00257%)
[SOURCE: NHTSA Traffic Safety Performance (Core Outcome) Measures For Wyoming]

Utah saw a large increase in traffic fatalities yet they did not legalize cannabis:

2012: 217 (Population: 2.855 million, 0.00760%)
2013: 220 (Population: 2.903 million, 0.00758%)
2014: 256 (Population: 2.943 million, 0.00870%)
2015: 276 (Population: 2.996 million, 0.00921%)
[SOURCE: Utah Fatal Crash Summary 2015]

So my conclusion is that although the answer to the question, does cannabis impair driving is yes, i feel it is much safer than alcohol and we should put more research into a method that could detect immediate impairment.


–Compton and Berning. DOT HS 812 117. Drug and Alcohol Crash Risk. U.S. Department of Transportation – National Highway Traffic Safety Administration. 2015.
–Elvik R. Risk of road accident associated with the use of drugs: a systematic review and meta-analysis of evidence from epidemiological studies. Accident Analysis Prevention. 2013. Review.
–Robbe and O’Hanlon. DOT HS 808 078. Marijuana and actual driving performance. U.S. Department of Transportation – National Highway Traffic Safety Administration. 1993.
–Robbe H. Marijuana’s effects on actual driving performance. HHMRC Road Research Unit, University of Adelaide. 1995.

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