Actual impairment should be tested, not just exposure to a drug.  My new app DRUID ("DRiving Under the Influence of Drugs") is in the App Store and uses neuropsych measures (e.g., reaction time, decision making) to calculate an overall impairment score.  People can now test their own level of impairment.

Our website is www.druidapp.com

After receiving my PhD from Harvard, I have been a professor at UMass/Boston for 40 years, with a specialty in research methods, measurement and statistics.

Michael Milburn
Psychology Department
UMass/Boston