The following is a news story from the Vincennes (Illinois) Sun-Times a couple of years ago:
Lawrenceville, Ill. Sepember 17, 2004 – The Lawrence County Board unanimously approved a three-year contract with union members of the sheriff's department that call for a pay raise and clarifies the blood-alcohol content an on-duty officer can have and continue to work Under terms of the contract, a deputy or other department employee can have a BAC level of up to .04 percent and remain on duty… Board member Gene Hays said he understood some people would oppose the .04 percent BAC level as too lenient. But he maintained there has to be a margin allowed for errors in testing; if the board tried to establish a zero-level standard, he said it would not stand up in court.
Interesting double standard. Drivers under 21 are subject to .01% "zero tolerance" laws in every state in the country — but cops with a .04% level can arrest them…. Meantime, MADD is busily pushing state legislatures to lower the adult level from .08% to .05%. Also of interest: prosecutors across the country tell juries that anyone over .05% is "probably" impaired. More interesting: The County Board has apparently been told by sheriffs that "there has to be a margin allowed for errors in testing" — at least for sheriffs. That's some margin: in a .08% case, that would be a 50% error! A suspect who tested .11% would actually be innocent. Funny how cops never testify to that in court…
(Thanks to Sam Peter Hopp and to E. Thomas Kemp, on whose blogsite the article appears.)