(a) The department shall defend any action and be responsible for all damages awarded therein in any court of this state against any county under existing laws whenever the cause of action accrues on a public road which at the time of accrual had been designated by the department as a part of the state highway system; provided, however, that no action may be brought under this Code section until the construction of the public road on which the injury complained of occurred has been completed and such public road has been officially opened to traffic as provided in subsection (b) of this Code section. When any such action is brought against a county in any court of this state, it shall be the duty of the plaintiff to provide for service of notice of the pendency of such action against the county upon the department by providing for service of a second original process, issued from the court where the action is filed, upon the commissioner personally or by leaving a copy of the same in the office of the commissioner in the Department of Transportation Building, Atlanta, Georgia. The service of process in such action upon the county shall not be perfected until such second original process has been served as provided in this Code section. The department shall also have the right and authority to defend, adjust, and settle in the name of such county and on its behalf any claim for damages for which the department ultimately may be liable under this Code section.

(b) A public road shall be officially opened to traffic within the meaning of this Code section on the date that the department gives written notice of final acceptance of such work to the contractor or political subdivision performing the work on such road or otherwise in writing acts so as to open the road to traffic by the general public.