NEWS Increase in car accidents?Blame the weather, the time of day — not legalize pot

Increase in car accidents?Blame the weather, the time of day -- not legalize pot
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Temporal patterns of human activity (such as yearly, weekly, and daily cycles) and severe weather are better predictors of vehicle accident experience than marijuana legalization, two North American actuarial organizations said in a media statement.

The Canadian Institute of Actuaries (CIA) and Casualty Actuarial Society (CAS) report, “Assessing the Impact of Cannabis Decriminalization on Vehicle Crash Experience,” analyzed the impact of cannabis decriminalization on vehicle crash experience in Canada and the United States. The study did not find any statistically significant effect of decriminalization on car crash mortality, frequency of insurance claims, or average cost per claim, especially over the long term.

In Canada, trends and seasonality in traffic accidents in Canada did not change significantly following the change in legal status, the actuarial group said in a media statement.

Likewise, the estimated statewide impact of decriminalization in the United States does not reveal any consistent, significant results to support the conclusion that decriminalization leads to an increase in road accidents or deaths.

The study included a review of past literature and new data analysis.

A review of the literature showed that while marijuana use does affect driving behaviour, this behavior is not always associated with a higher risk. For example, in a 2016 study, impaired drivers exposed to THC had slower speeds and longer following distances, suggesting drivers are aware of the impairment and are more cautious. In contrast, an earlier study using fatal accident data from 1993 to 2003 (National Highway Traffic Safety Administration’s Fatality Analysis Reporting System) found that among drivers aged 20-50 who tested positive for marijuana 5 percent had a higher odds ratio for exhibiting so-called “driver-related factors” — factors coded in police reports, including speeding, erratic maneuvering and failure to yield or obey traffic signs.

See related article, “CBD vs. THC: What’s the Difference?”

In addition to these studies investigating the effects of marijuana use on driving ability, another set of previous studies has looked at the effect of decriminalization on the proportion of drivers who test positive for marijuana use. Actuarial studies belong to the third group of studies, which mainly focus on estimating the combined impact on the number of accidents and fatalities.

Previous studies of this last category had shown mixed results, the actuarial report said, documenting seven of those studies and concluding that most found no effect of cannabis legalization on road crashes.

The actuarial report indicated that these prior studies had some flaws. There are data limitations in some previous studies, which considered only one region. Others rely on only one statistical method. “Some other publications consider relative changes without overall effects,” the report said, citing a 2015 report that looked at the proportion of drivers who tested positive for THC, but not changes in the number or severity of crashes .

The new actuarial data analysis, using more novel methods and machine learning techniques, did not detect a statistically significant sustained effect of marijuana legalization on accident experience.

“The methods used in this study include improved statistical models, machine learning and other data science techniques. These models use high-resolution weather data to explain the effects of weather factors,” report author Dr. Vyacheslav Lyubchich said in a media statement .

In addition to providing new insights into the impact of marijuana legalization on vehicle accident experience and on the insurance industry, actuaries set out to develop a systematic approach to assessing similar types of changes in the future, such as the legalization of other substances or the adoption of auto drive a car.

Some of the steps in the method are summarized below:

  1. Establish relevant “controls” to quantify the impact of cannabis decriminalization. Controls can be the same region (before and after decriminalization, if there are quality time series of sufficient length for both periods) or similar regions for simultaneous comparison.
  2. Collect crash data, including frequency, severity and type of crashes. Gather information on confounding conditions such as weather, time of day and day of the week, driver age and gender.
  3. Applying statistical techniques to quantify the effects of cannabis legalization while controlling for confounding factors.

The statistical model used data from Canada and the United States for 2016-2019, including official reports on private vehicle crashes and losses in Canada, fatalities, and weather factors in the United States.

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