In 2003, William D. Healan, III, along with co-counsel, Sherry Boston, argued Cooper v. State in front of the Georgia Supreme Court. On October 6, 2003, the Court issued a landmark decision which declared a portion of Georgia's DUI law unconstitutional. Because of the Cooper decision, the police in Georgia can no longer, as a matter of standard procedure, request a blood, breath, or urine test from a motorist involved in a accident with a fatality or serious injury. The Atlanta Journal-Constitution ran a front page headline after the Cooper decision declaring "Part of Georgia's DUI Law Erased".

In January 2004, Mr. Healan was awarded "Best Case of the Year" by the Georgia Defenders of Drinking Drivers (D.O.D.D.) group for Cooper v. State.

In 2000, the Georgia Supreme Court decided another of William D. Healan, III's cases. In King v. State, the Supreme Court held a different Georgia statute unconstitutional as it applied to Mr. Healan's client. The Court held that a District Attorney's office had violated Mr. Healan's client's right to privacy when it subpoenaed her medical records to charge her with DUI based on her hospital blood test. The King case changed the way that hospitals throughout Georgia responded to medical records requests and subpoenas.

In 1997, the Georgia Court of Appeals decided Mr. Healan's case, State v. Frazier. Frazier was one of the first cases in Georgia that prevented prosecutors from charging motorists with drug possession when their DUI blood or urine test came back positive for an illegal drug.

William D. Healan, III has spoken at several seminars designed to educate other attorneys in the fields of criminal and DUI law. October 1998, Georgia Association of Criminal Defense Lawyers fall seminar - Criminal Discovery.  October 2003, Georgia Indigent Defense Counsel - Ethically Defending Co-Defendants in a Drug Case. February 2004, Using the ALS hearing to Win/Limit Issues at Trial.