I think you are drawing the false line with your assumption that the legally-dictated .08 BAC or even .15 automatically equates to a dangerous level of impairment for everyone. Some sort of driving skill field test would be a more effective and fair measure of driving impairment for drinkers, bluehairs, idiots, etc.
So you want all people to come to the DMV hammered and drive a closed course to establish how much they can drink and still drive? Do you really find this possible?
.08 is established thru study to be impairment and all 50 states have adopted that level as ground zero.