It is a better standard than measuring intoxicant levels, because it actually PROVES a danger. It is a reasonable expectation that the government prove a charge, no?
So, the cops can't just pull over your "stoners" for a dim tail light and then measure some arbitrary juice level & stick them for ten grand in penalties based on that. Cry me a river.
You are right. What did they do before the breathalizers? They did roadside impairment tests. Walk the line. Touch your nose. Count. Say the ABC’s. Go back to that.
observe the obviously impaired drivers and cite them
That is a rather subjective standard and it gives law enforcement more discretion than I am comfortable with.
In fact I was pulled over once for a roadside sobriety test at a USAF gate. I didn’t do well (drunk buddy just woke me up from a sound sleep to help him find his van). The pocket breathalizer read 0.00. Under your standards I’d have been given a DUI for my buddy being passed out drunk!
Oddly though, that was the only time I ever went through that gate sober.