A new study by economists at the University of Colorado Denver and Montana State University reveals that so-called "per se" drugged driving laws have no discernible impact on traffic fatalities.
Per se laws set thresholds for controlled substances above which drivers are considered impaired.
Since 1990, 11 states have passed zero-tolerance drugged driving laws making it illegal to drive with detectable levels of a controlled substance in the system. Five other states have passed similar laws specifying nonzero limits for controlled substances or their metabolites.
"These laws are intended to make the job of prosecuting drugged drivers easier," said Daniel Rees, professor of economics at the University of Colorado Denver who co-authored the study with D. Mark Anderson, assistant professor of economics at Montana State University. "In states without these laws, prosecutors must rely on field sobriety tests or evidence that a motorist was driving erratically in order to prove impairment."
The Office of National Drug Control Policy (ONDCP) recently announced a goal of reducing drugged driving by 10 percent within three years. In an effort to achieve this goal, the ONDCP is encouraging all 50 states to prohibit driving with detectable levels of a controlled substance in the system.
Although there is anecdotal evidence that the new drugged driving laws make prosecution easier, this is the first study to examine their effectiveness.
Using state-level data from the Fatality Analysis Reporting System (FARS) for the period 1990-2010, Anderson and Rees examined the relationship between adopting controlled substance thresholds for drivers and traffic fatalities. They found that the relationship is statistically indistinguishable from zero and concluded that there is no evidence that these limits reduced traffic deaths.
"Our study is particularly timely given that Washington voters recently passed Initiative 502, which legalized the recreational use of marijuana but prohibited driving with THC levels equal to, or greater than, five nanograms per milliliter of blood." Anderson said. "Setting a THC standard for drivers may, in the future, be viewed by voters as a necessary complement to legalizing marijuana for recreational or medicinal use."
The FARS data represent a census of all fatal injuries resulting from motor vehicle accidents in the United States and include information on when the accident took place. Using this data, Anderson and Rees distinguished between nighttime and daytime traffic fatalities. They also distinguished between weekend and weekday traffic deaths.
Although the percentage of drivers testing positive for marijuana and other controlled substances is highest during the night and on weekends, they found no evidence that these laws, which have been adopted by 16 states, led to a reduction in traffic fatalities at either time.
"There is strong evidence that drivers under the influence of marijuana have slower reaction times than drivers who are not under the influence of marijuana," Rees said. "As currently implemented, these laws have no discernible impact on traffic fatalities."
The study, which is under review, is available as an IZA working paper at: www.iza.org/en/webcontent/personnel/photos/index_html?key=4915 .
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