"BAC--whatever it is--isn't an adequate tool to determine whether or not a person is impaired."
Then what is?
What do you poseas far as law, that would apply to all people equally?
I'd love to hear it. You say it is 'dumb' but I do not see you offer an alternative.
"You want to make airplanes safer? Let people carry guns on the airplane"
Is that what EL AL does? When was the last time one of their airplanes was hijacked?
I notice you offer an alternative for airplanes (though I disagree with it) where is the alternative for BAC?
I'm sorry, I thought the alternative was plainly obvious. Instead of setting probable cause for arrest at an arbitrary number, simply let police use their discretion to arrest who they deem to be impaired drivers. Policemen are supposedly trained to do this, and police arrest people all the time based on their own opinion as to what amounts to probable cause.
Let the policeman make the arrest, let the jury determine whether the person was or wasn't impaired. Pretty simple. It's what policemen do every day.