Ethan Siegel and Alex Berezow are right to identify marijuana-impaired driving as a significant national highway safety problem, but no equivalent will ever exist for marijuana or any other drug like the familiar 0.08 BAC for alcohol (“We Need a DUI Test for Marijuana,” op-ed, Aug. 23).

This is because, unlike for alcohol, for other drugs there is no close relationship between tissue levels and impairment. The search for a tissue level that defines impairment for marijuana or other drugs is illusory. Moreover, it is not possible...