The NRS researchers noted that “Drug presence does not necessarily imply impairment.” For many drug substances, other studies have shown that drug presence can be detected after a driver is no longer impaired. For example, traces of marijuana use can be detected in blood samples several weeks after heavy chronic users stop using it.

Survey 2: Does marijuana use increase risk of car crashes?

In a related drug and alcohol crash risk study, NHTSA investigated the effects of alcohol, drugs and the combination of alcohol and drug use by drivers on crash risk. The agency reviewed police-reported crashes, looking specifically at whether marijuana use by drivers leads to a greater risk of crashes, noting that the trend toward legalization of marijuana for medical and recreational use have increased concerns over the potential risks of impaired driving.

The crash risk study collected data from more than 3,000 drivers who were involved in crashes that were reported to the policy and 6,000 control drivers, not involved in crashes. It confirmed that alcohol use by drivers was clearly associated with an increased risk of being in a crash. Drivers with a BrAC of 0.08 had about four times the risk of crashing as sober drivers while drivers with a BrAC of 0.15 had 12 times the risk.

When looking at crash-involved drivers who had used drugs, this study, like the roadside survey, found that marijuana was the drug most frequently used, detected in 7.6% of the crash-involved drivers and 6.1% of the control drivers. In contrast, alcohol was detected in 5% of the crash-involved drivers and 2.7% of the control drivers.

The study also found the following drugs and percentages:

• Narcotic-analgesics (opiates) in 3.4% of crash-involved drivers and 3% of the control drivers
• Stimulants in 3.8% of crash-involved drivers and 3.6% of the control drivers
• Sedatives in 2.9% of crash-involved drivers and 2.3% of the control drivers

Crash-involved drivers were significantly more likely to test positive for THC and sedatives, to have used more than one class of drug, and to have used any type of drug than were control drivers. More of the drivers involved in crashes tested positive for illegal drugs at 10.4% than the control drivers (8.8%).

In the crash study 9.4% of the weekend nighttime control drivers were positive for THC. The higher drug prevalence at nighttime strongly suggests to the researchers that recreational use is a significant component of overall drug use.

Noting that marijuana users are more likely to be involved in accidents, by about 25%, the study also suggested that the increased risk may be in part because marijuana users are more likely to be young men—a group that has a higher risk of crashes than the general population. According to the study, other variables such as age, gender, ethnicity and alcohol use, were highly correlated with drug use and account for much of the increased risk associated with the use of illegal drugs and THC.

Interestingly, the study found that there was no difference in crash risk for THC-positive drivers who were also positive for alcohol than for THC-positive drivers with no alcohol, beyond the risk attributable to alcohol. The researchers, who reviewed similar studies in their research report, noted that other studies using driving simulators and test tracks have found that marijuana at sufficient dosage levels will affect driver risk.

According to the crash risk study, “While the findings of this case control study were equivocal with regard to the crash risk associated with drug use by drivers, these results do not indicate that drug use by drivers is risk-free.”

Future studies planned

To further understand the risk of impaired driving, NHTSA plans a roadside survey in Washington State to assess risk in a state that recently legalized marijuana. The agency also plans a simulator study with the National Institute on Drug Abuse to assess how drivers under the influence of drugs behave behind the wheel.

“Drivers should never get behind the wheel impaired, and we know that marijuana impairs judgment, reaction times and awareness,” said Jeff Michael, NHTSA’s associate administrator for research and program development. “These findings highlight the importance of research to better understand how marijuana use affects drivers so states and communities can craft the best safety policies.”