Canadian researchers have discovered smoking marijuana three hours before driving can more than double a driver's chance of being involved in a serious crash. The study examined data collected from 49,111 victims who had been seriously injured or died in an accident. Researchers specifically looked at cases where tetrahydrocannabionol, the active compound in marijuana, was found in the victims' blood stream but where other drugs and alcohol were absent. The study also evaluated instances where the driver admitted to smoking within three hours of the crash. The findings demonstrate that, while those who drive under the influence of alcohol are still more likely to be involved in a collision than their pot-smoking counterparts, marijuana significantly impairs an individual's ability to safely operate a vehicle.
The study did not note precise levels of THC present in each instance, though as CBC News reports, other research has proven a positive link between the higher crash likelihood and elevated THC levels.
Law enforcement agencies are currently struggling to effectively test for THC impairment during roadside stops. Unlike blood-alcohol levels, which can be easily tested for using a breath test, THC is harder to detect. As a result, most agencies have adopted a zero-tolerance policy.