Police decide who ingested exactly which drugs based on a 12-step collection of clues gathered by staring into suspects’ eyes, pinching their muscles, taking their blood pressure and watching closely how they walk and talk and balance.
They are safer than an asswipe glued to their Galaxy or iPhone.
The amount of people I see visibly fucking with their phones while driving (often at high speed) is really unsettling. In my day to day driving, I'm far more concerned about people on their phones than I am people stoned. They represent different hazards, but stoned drivers tend to be much more predictable.