Decades of scientific research eventually led to the establishment of .08 percent as the nationwide blood-alcohol limit for drivers. However, even now, testing for alcohol impairment is far from a perfect science. Samples can be tainted or otherwise compromised by flawed testing and procedural mix-ups, and standardized field sobriety tests are still largely subjective.
Now significant research is being done on how cannabis use affects drivers, the goal being to establish guidelines for what is regarded as driver impairment and what is regarded as safe, reasonable use. The federal government funded a years-long experiment in which volunteers consumed controlled amounts of marijuana and alcohol and then got behind the wheel of a driving simulator. According to a chief researcher on the project, the volunteers "were happy to participate."