There is no federal limit that each state uses for driving under the influence of cannabis, as there is the .08% BAC (blood alcohol content) for alcohol. Therefore the rules that determine what is “stoned driving” vary widely from state to state.
Cannabis is a particularly difficult substance for which to measure intoxication because its effects on driving differ from the effects of alcohol, and they vary from person to person depending on one’s tolerance. Cannabinoids also remain in a person’s system long after s/he is impaired. Despite that, some states require a urine test from drivers, while a blood test is the more accurate way to measure intoxication; this means that someone could be sent to jail for having consumed weeks before hitting the road.
Washington and Colorado tackled this when they were the first two states to legalize cannabis for personal use. By looking at existing–but very lacking–research, Washington state first set their limit for cannabis in a person’s system at 5 ng/ml (of blood). Some studies would suggest 5 ng/ml is too low, while others would suggest it’s too high. For now, this appears to be a good guesstimate, and Washington has set aside some tax proceeds from cannabis sales to study this topic in hopes of determining a more precise limit.
In February 2015, the National Highway Traffic Safety Administration released reports about impaired driving and found that the number of drivers with THC in their system has increased since 2007. It is clear that intoxication at a certain threshold will impair drivers, they note, but additional research needs to be done to determine the exact effects of cannabis.
Studies to-date have not been consistent, according to the latest NHTSA report: “While a number of previous studies have shown some increased risk associated with marijuana use by drivers, many studies have not found increased risk.” (Read that full report here.)
An NHTSA spokesperson told Cannabis Wire, “Further progress, particularly in the area of marijuana and driving, will require new research and a better understanding of how the drug affects individuals and how these effects translate into driving performance and traffic risk.
Conversely, The AAA Foundation for Traffic Safety commissioned some studies on cannabis on the road and found that “Fatal crashes involving drivers who recently used marijuana doubled in Washington after the state legalized the drug.” A similar increase was found in Colorado, but officials say that the role of legalization in the increase is unclear as the effects of cannabis on drivers is still in need of more research.