No ordinance, or franchise shall become operative until 30 days after its passage by the Council and approval by the Mayor, except measures necessary for the immediate preservation of the peace, health or safety of the town, but such an emergency measure shall not become immediately operative unless it states in a separate section the reason why it is necessary that it should become immediately operative, and unless it is approved by the affirmative vote of 3/4 of all the members elected to the Council, taken by ayes and nays.
(A.R.S. § 19-142B)  (1986 Code, § 2-5-6)