Clearly drunk driving is a major risk factor for traffic accidents; however, the data on marijuana impairment is less persuasive.
Two National Highway Traffic Safety Administration (NHTSA) studies seem to come to different conclusions. Studies Twenty three states have legalized marijuana (four states and the District of Columbia for recreational use). Perhaps as many as ten other states will follow their lead within the next three hundred and sixty five days.
A second NHTSA report based on more than 3,000 crash-involved drivers and 6,000 drivers not involved in crashes revealed that drivers who tested positive for THC were 25 percent more likely to be involved in a crash. However, other factors like age, gender, ethnicity and blood alcohol concentration were considered statistically more likely to increase the risk of an accident. NHTSA researchers concluded, “Caution should be exercised in assuming that drug presence implies driver impairment.”
Measuring impairment in a person using marijuana isn’t proved by THC levels. “Most psychoactive drugs are chemically complex molecules, whose absorption, action and elimination from the body are difficult to predict,” NHTSA writes, "and considerable differences exist between individuals with regard to the rates at which these processes occur. Alcohol, in comparison, is more predictable.” More importantly, marijuana users can have THC in their bodies days or even weeks after using the drug, long after any psychoactive effects. For example, a U.S. district court in New Jersey, excluded evidence that a driver had used marijuana the night before an accident “because its probative value is weak, and a jury is likely to be inflamed by evidence of use of intoxicating substances.” The defense presented evidence that the driver was "significantly impaired... since a significant amount of time passed between the marijuana use and the accident, and there is no standard for measuring marijuana impairment.”