The DRUIDapp is widely used in the scientific community to study impairment.  Here are some of the research studies using the app as the measurement tool.

University of SydneyDr. Danielle McCartneyCBD and Driving Impairment
Yale UniversityDr. Godfrey PearlsonDRUID and driving
Univ of KentuckyDr. Shanna BabalonisAlcohol, opioids and cannabis
Johns Hopkins Medical InstituteDr. Kelly DunnOpioid impairment
Johns Hopkins Medical InstituteDr. Tory Spindle & Dr. Elise WeertsAlcohol and cannabis in combinations
Johns Hopkins Medical InstituteDr. Ryan VandreyDRUID app–demonstration of proof of concept
University of Colorado, School of Public HealthDr. Cinnamon BidwellFrequent cannabis users, high-potency cannabis
Washington State UnivDr. Nick LovrichGreen Labs; Frequent cannabis users study
Brown UnivDr. Jane MetrikAlcohol consumption after cannabis