At the Lifesavers National Conference on Highway Safety in Louisville, KY earlier this year, there was a session on Standard Field Sobriety Tests (SFSTs) and drug impairment. The session was about how research shows that SFSTs are effective at detecting impairment due to drugs and not just due to alcohol.
Here is some of the research that was provided in the session:
An Examination of the Validity of the Standardized Field Sobriety Test in Detecting Drug Impairment
The Standardized Field Sobriety Tests and Measures of Cognitive Functioning