(a) The powers and duties of a regional mobility authority conferred by this chapter are public and governmental functions exercised for a public purpose and for matters of public necessity.
(b) The exercise of the powers and the performance of the duties by a regional mobility authority under this chapter are immune from suit in tort unless immunity is expressly waived in writing.
(c) A regional mobility authority is not immune from suit in contract if the contract arises from the performance of duties authorized by law.