Why Studies Using the Same Crash Databases Reached Opposite Conclusions About Marijuana and Fatal Accidents
Two major studies using the same U.S. crash databases (FARS and NRS) reached contradictory conclusions about marijuana and crash risk, primarily because of inconsistent drug testing practices and database limitations.
Quick Facts
What This Study Found
The authors examined why two studies using similar databases and approaches produced opposite conclusions about marijuana's contribution to fatal crash risk. Li et al. (2013) found marijuana significantly increased crash risk, while Romano et al. (2014) found no significant increase after controlling for confounders.
The key source of divergence was limitations in the FARS (Fatality Analysis Reporting System) database. Drug testing of fatally injured drivers is not standardized across jurisdictions: different states test for different drugs at different rates, using different methods, and testing rates have changed dramatically over time. These inconsistencies mean that the pool of tested drivers is not representative of all drivers in fatal crashes.
The authors conclude that FARS should not be used to examine trends in drug use prevalence among crash-involved drivers or to obtain precise risk estimates. However, under certain conditions (data from jurisdictions that routinely test, with consistent procedures), it could provide relative risk comparisons.
Key Numbers
Two studies compared: Li et al. 2013 (found significant crash risk from marijuana) vs. Romano et al. 2014 (found no significant risk). Both used FARS and NRS databases. Testing rates and methods varied widely across jurisdictions and over time.
How They Did This
Methodological comparison of two published case-control studies that used FARS and NRS databases. The authors identified specific analytic choices and database limitations that led to contradictory results.
Why This Research Matters
Policy decisions about cannabis and driving are heavily influenced by epidemiological research. If the major databases used for this research produce contradictory results depending on analytic choices, policymakers need to understand these limitations. This paper serves as a critical methodological caution for all drugged driving research.
The Bigger Picture
The cannabis-and-driving debate is complicated by the fundamental difficulty of measuring impairment. Unlike alcohol, where blood levels correlate reasonably well with impairment, THC blood levels do not reliably indicate whether someone is currently impaired. When the databases used to study the question also have significant limitations, reaching firm conclusions becomes extremely challenging.
What This Study Doesn't Tell Us
This is a methodological critique, not new empirical research. The authors do not provide definitive answers about whether marijuana increases crash risk, instead highlighting why existing studies disagree. Their own prior work (Romano et al. 2014) is one of the two studies being compared, creating potential bias.
Questions This Raises
- ?Can U.S. crash databases be improved to consistently test all fatally injured drivers for drugs?
- ?What is the best study design to determine marijuana's true contribution to crash risk?
- ?Should policy rely on laboratory impairment studies rather than epidemiological crash data?
Trust & Context
- Key Stat:
- Same databases, opposite conclusions: database limitations in drug testing are the primary reason
- Evidence Grade:
- Moderate evidence: methodological critique illuminating limitations of the U.S. crash database infrastructure.
- Study Age:
- Published in 2017. Crash database limitations remain an ongoing challenge for drugged driving research.
- Original Title:
- Marijuana and the Risk of Fatal Car Crashes: What Can We Learn from FARS and NRS Data?
- Published In:
- The journal of primary prevention, 38(3), 315-328 (2017)
- Authors:
- Romano, Eduardo(6), Torres-Saavedra, Pedro, Voas, Robert B(3), Lacey, John H
- Database ID:
- RTHC-01504
Evidence Hierarchy
Summarizes existing research on a topic.
What do these levels mean? →Frequently Asked Questions
Does marijuana actually increase crash risk or not?
This paper shows that the answer depends partly on which database limitations you account for. Laboratory studies consistently show marijuana impairs driving skills, but epidemiological studies using real-world crash data have produced conflicting results, partly due to inconsistent drug testing practices across jurisdictions.
Why is it so hard to study marijuana and driving?
Unlike alcohol, there is no simple blood test that reliably indicates current marijuana impairment. THC metabolites can persist in the body for days or weeks. Additionally, drug testing in crash investigations is not standardized across U.S. states, creating gaps and biases in the data used for research.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-01504APA
Romano, Eduardo; Torres-Saavedra, Pedro; Voas, Robert B; Lacey, John H. (2017). Marijuana and the Risk of Fatal Car Crashes: What Can We Learn from FARS and NRS Data?. The journal of primary prevention, 38(3), 315-328. https://doi.org/10.1007/s10935-017-0478-3
MLA
Romano, Eduardo, et al. "Marijuana and the Risk of Fatal Car Crashes: What Can We Learn from FARS and NRS Data?." The journal of primary prevention, 2017. https://doi.org/10.1007/s10935-017-0478-3
RethinkTHC
RethinkTHC Research Database. "Marijuana and the Risk of Fatal Car Crashes: What Can We Lea..." RTHC-01504. Retrieved from https://rethinkthc.com/research/romano-2017-marijuana-and-the-risk
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.