I can't prove this, but, if it were possible to know, I think we'd find out that most of the alcohol related deaths were caused by (drumroll)  Alcoholics.  For these people, their alcohol/blood content means nothing. There going to drive till they die, or kill some one.
  Lowering the percentage and arresting non-alcoholic is a 'Feel Good' but it does nothing to reduce the accident rate.   In addition, it's a 'No Brainer' for a politician because who would dare be against lower limits.
   I'm not really upset about this, it's just how things work.