(a) The Commission may, after hearing, upon notice, by order in writing, fix just and reasonable individual rates, joint rates, charges or schedules thereof, as well as commutation, mileage and other special rates, which shall be imposed, observed and followed thereafter by any public utility whenever the Commission determines any existing individual rate, joint rate, toll, charge or schedule thereof, or commutation, mileage, or other special rate to be unjust, unreasonable, insufficient, or unjustly discriminatory or preferential.
(b) No order of the Commission requiring a change in rates shall become operative until at least 30 days after service thereof except upon the written consent of the public utility affected.