Using DUIs as an example, these cover things besides alcohol, can be prescription drugs, probably other things that impair motor function (IANAL).

Roadside DUI test is a general test for motor impairment. BAC tests are a nice proxy for that in the case of alcohol, and a legally convenient one as they are precise and objective, but the police care (morally speaking) about your motor impairment not your BAC.

tl;dr, we can already answer the question "is this person too stoned to drive?", roadside DUI test.