Study Finds Legalization Correlates With Safer Roads ... Wait, What?



A controversial new study by two economists indicates that medical marijuana legalization laws have resulted in a nearly 9 percent drop in traffic deaths, and a 5 percent reduction in beer sales.

It's a contrarian case of Freakonomics that's sure to invite scrutiny. The secondary author — University of Colorado Denver professor of economics Daniel Rees — is the first to admit he didn't expect the results, but he also stands by them.

“If critics want to collect the data themselves, boy, the results will jump out at them the way they jumped out at us,” he said. “Our results are pretty robust. We're pretty confident they're going to hold up.”

Less crashes?
  • Less crashes?

Rees, along with D. Mark Anderson, an assistant professor of economics at Montana State University, used government statistics on road fatalities and drug behavior to look at what happened in thirteen medical marijuana states between 1990 and 2009.

They found the safety effect of legalization is comparable to gains from increasing the legal drinking age to 21, or mandating seat belts for those ages 14 to 18. The safety effect is even greater in the groups most at-risk for road fatalities: young, male, intoxicated, weekend night drivers.

“Legalization is associated with a 19-percent decrease in the fatality rate of 20-29 year-olds,” the study states.

That was not Rees' hypothesis. “Include us in that group who thought, 'This is going to be crash central,'” he said. “We were totally surprised when the results came out so strong in the other direction.”

There was also an attendant drop in drinks consumed. Using data from the Behavioral Risk Factor Surveillance System, the study finds that medical marijuana laws "are associated with decreases in the number of drinks consumed, especially among 20- through 29-year-olds."

“Using data from the Beer Institute, we find beer sales fell after [legalization] comes into effect, suggesting marijuana substitutes for beer, the most popular alcoholic beverage among adults.”

The drop in drinking and the safety effect in the groups most associated with alcohol-involved road fatalities was so large “that a lightbulb went off,” Rees said. “'Oh, what's going on? People are switching from alcohol to marijuana'.”

Rees speculates that young males are the early adopters to changing medical marijuana law. As the overall climate of prohibition cools in a state, they're choosing weed instead of booze. “My suspicion is as soon as you open the door to individuals growing [medical] marijuana, you've opened the door to diversion to the recreational markets.” And, well, weed is safer, studies show and the paper asserts.

This is not going to play well with the wannabe Attorney Generals and narcotics union lobbyists of the world. The oldest and most reliable rebuttal to legalization is the specter of “road carnage”, and critics will have a number of attacking points.

Rees and Anderson had not worked with the Fatal Accident Reporting System before this one-year-long study, Rees said. However, they both have experience studying risk behavior among youth: Anderson worked on explaining methamphetamine's decline in other states and Rees has worked on papers about marijuana and alcohol use. The study was funded by the Institute for the Study of Labor in Bonn, a research center focused on labor economics and policy. The study is not peer-reviewed, Rees said. He and Anderson published it as a working paper for a number of reasons, including getting it out first (he said a number of researchers are onto the topic).

“If our critics wanted to call it a work in progress our answer would be, 'Yes',” he said.

Another problem with the study is the sheer number of different flavors of legalization there are in America. Vermont's 300-patient program is very different from California's several-hundred-thousand-patient scene. The study does not capture the differences between states with voluntary and compulsory registration, qualifying medical conditions and an official dispensary system. Rees says they weighted states by population and results are overall averages. Critics will also question if there's enough pot users to account for the drop in deaths, Rees said. There are, he says.

Lastly, correlation does not equal causation, of course, so the team used a difference-in-difference analysis to account for broader national and state trends — like the recent reduction in road fatalities nationwide — Rees said. But even so, “traffic fatalities fall by nearly 9 percent after the legalization of medical marijuana,” the study says.

“Clearly policy makers, the public, concerned mothers, we all want roads to be safer,” said Rees. “I do think there's a certain irony in the fact that if it turns out that, indeed, medical marijuana is driving a reduction in traffic fatalities. Often policy is not driven by data or logic. It this study can inform policy in a positive way I'd be happy.”

Here is a .pdf of the study from the Institute for the Study of Labor. We'll follow up when it and others like it get published.

Some more excerpts below:

Marijuana was introduced in the United States in the early-1600s by Jamestown settlers who used the plant in hemp production; hemp cultivation remained a prominent industry until the mid-1800s (Deitch 2003). During the census of 1850, the United States recorded over 8,000 cannabis plantations of at least 2,000 acres (U.K. Cannabis Campaign 2011). Throughout this period, marijuana was commonly used by physicians and pharmacists to treat a broad spectrum of ailments (Pacula et al. 2002). From 1850 to 1942, marijuana was included in the Unite States Pharmacopoeia, the official list of recognized medicinal drugs (Bilz 1992). In 1913, California passed the first marijuana prohibition law aimed at recreational use (Gieringer 1999); by 1936, the remaining 47 states had followed suit (Eddy 2010). In 1937, The Marihuana Tax Act effectively discontinued the use of marijuana for medicinal purposes (Bilz 1992), and marijuana was classified as a Schedule I drug in 1970.1 According to the Controlled Substances Act (CSA), a Schedule I drug must have a “high potential for abuse,” and “no currently accepted medical use in treatment in the United States” (Eddy 2010).

Laboratory studies have shown that cannabis use impairs driving-related functions such as distance perception, reaction time, and hand-eye coordination (Kelly et al. 2004; Sewell et al. 2009). However, neither simulator nor driving-course studies provide consistent evidence that these impairments to driving-related functions lead to an increased risk of collision (Kelly et al. 2004; Sewell et al. 2009). Drivers under the influence of marijuana reduce their velocity, avoid risky maneuvers, and increase their “following distances,” suggesting compensatory behavior (Kelly et al. 2004; Sewell et al. 2009). In addition, there appears to be an important learning-by-doing component to driving under the influence of marijuana: experienced users show substantially less functional impairment than infrequent users (Sutton 1983).

Like marijuana, alcohol impairs driving-related functions such as reaction time and hand-eye coordination (Kelly et al. 2004; Sewell et al. 2009). Moreover, there is unequivocal evidence from simulator and driving-course studies that alcohol consumption leads to an increased risk of collision (Kelly et al. 2004; Sewell et al. 2009). Even at low doses, drivers under the influence of alcohol tend to underestimate the degree to which they are impaired (MacDonald et al. 2008; Marczinski et al. 2008; Robbe and O’Hanlon 1993; Sewell et al. 2009), drive at faster speeds, and take more risks (Burian et al. 2002; Ronen et al. 2008; Sewell et al. 2009). When used in conjunction with marijuana, alcohol appears to have an “additive or even multiplicative” effect on driving-related functions (Sewell et al. 2009, p. 186), although there is evidence that chronic marijuana users are less impaired by alcohol than infrequent users (Jones and Stone 1970; Marks and MacAvoy 1989; Wright and Terry 2002).5

For instance it is possible that legalizing medical marijuana reduces traffic fatalities though its effect on substance use in public. Alcohol is often consumed in restaurants and bars, while many states prohibit the use of medical marijuana in public.35 Even where it is not explicitly prohibited, anecdotal evidence suggests that the public use of medical marijuana can be controversial.36 If marijuana consumption typically takes place at home, then designating a driver for the trip back from a restaurant or bar becomes unnecessary, and legalization could reduce traffic fatalities even if driving under the influence of marijuana is every bit as dangerous as driving under the influence of alcohol.