Lupinus is certainly right that BAC isn't as objective some people pretend it to be *as a measure of performance impairment,*, BUT the nice thing about BAC is -- barring bad testing, which I won't say is impossible or uncommon, but the procedure in Philadelphia County seems to me pretty well implemented -- that it's objective numerically at least. That does mean that some people who would be sober enough to drive would be legally barred from doing so, and that's far from ideal. Still, to the degree that is BAC is predictable by the drinker based on time / body weight / amount consumed (to a reasonable degree of certainty at least), it strikes me as better than relying on subjective / observational techniques alone.
timothy
Why?
If you are checking the BAC one of 2 things has occured.
You have either witnessed them driving in a manner which would justify a stop, which is subjective but should rise to the level of a violation. If it's a violation, punish the violation.
-or-
You've set up at a checkpoint and are stopping cars "randomly". In that situation, absent witnessing them driving, again subjectively, in such a manner as to violate traffic laws, there is NO reason to assume them to be a danger to themselves or others. Why then bother checking them against an arbitrary number that, apparently, is not relevent to that particular person's ability to drive?
If folks aren't breaking the law, they need to be left alone.