[MaxMax]

So they sampled 3000 people in accidents, and said they wouldn't cause
accidents if they were stoned?

But they were already in an accident to participate in the study!
Did I misread something?

[ConservingFreedom]

There was a control group:

“One week after a driver involved in a crash provided data for the study, control drivers were selected at the same location, day of week, time of day, and direction of travel as the original crash. This allowed a comparison to be made between use of alcohol and other drugs by drivers involved in a crash with drivers not in a crash, resulting in an estimation of the relative risk of crash involvement associated with alcohol or drug use.”

http://www.nhtsa.gov/staticfiles/nti/pdf/812117-Drug_and_Alcohol_Crash_Risk.pdf