The decision to dismiss a case is usually made by the prosecutor that eventually receives the case.  The first thing a prosecutor will do when they review a case is see if the basis for the stop was legal.  If the initial stop is illegal—then the case usually gets dismissed.  The next thing they will look at is jurisdiction—i.e. was the officer in their jurisdiction and if not, did they have a justifiable exception.  If the officer did not have a basis for being out of their jurisdiction, then they case will be dismissed.  The third review step is the gathering of evidence.  If there is a problem with how the evidence of intoxication was gathered (for example, very poor administration of field sobriety tests such that the results were invalid), then the case will often be dismissed.  The final review is the sufficiency of the evidence.  If the evidence does not show that the person was intoxicated beyond a reasonable doubt, then the prosecutor can make the decision to drop the case.   A fifth potential reason for dismissal is the credibility of an officer.  Sometimes a DA’s office will have experiences with an officer such they feel like the officer’s credibility cannot be rehabilitated.  When this happens, they will start to reject all of this officer’s cases.Of all the reasons listed above, the two most frequently used reasons for dismissal are illegal stop and insufficient evidence.  This is related mostly the point of view and objectives of the officer on the street versus the prosecutor in the court room.  When an officer is on the side of the road, they have to make the best decision they can.  Even if a DUI is a close call case, they will often arrest the person to avoid a headline that reads “Cop let DUI driver go before fatality,” in the event that the driver hurts someone else.  The amount of evidence needed to conduct an arrest is much lower than the amount of evidence needed to support a conviction.  When a prosecutor is reviewing the case, they are looking to see if the evidence supports a conviction beyond a reasonable doubt.  If the case, in a hind sight view, does not have enough evidence to meet the second, higher standard, then the case is usually dismissed.