The department may, for the protection of the State of California, insure any bridge on the State Highway System acquired or constructed under provisions of the California Toll Bridge Authority Act against all risks in any amount up to the full insurable value thereof. In case of damage or destruction, the proceeds of such insurance shall be applied to the restoration of such bridge. Any such bridge may be so insured irrespective of the retirement of all indebtedness incurred for its acquisition or construction, and the cost of such insurance shall be paid from any funds available to the department for highway purposes.

Any such

bridge, other than bridges over the San Francisco Bay, shall, after retirement of all indebtedness incurred for its acquisition or construction, be maintained and operated as a free bridge and the cost thereof charged to funds available for maintenance of state highways.