There have been no studies. Based on anecdotal evidence and my own evidence from dealing with drivers, I would suggest you're talking about 85% to 90% of the people who do sobriety tests, whether it's for alcohol or drugs, going back to the police station for further investigation.
The numbers of people who do sobriety tests and totally pass them are very slim, because you've got something there to start with to get them to come out and do the tests. We're not just stopping people and randomly saying, “I want you to do these tests.” There has to be something there to start with.