(a) It shall be the duty of every contract carrier by motor vehicle to file with the commission, publish and keep open for public inspection, in the form and manner prescribed by the commission, schedules or, in the discretion of the commission, copies of contracts containing the minimum charges of such carrier for the transportation of passengers or property in intrastate commerce in this state and any rule, regulation or practice affecting such charges and the value of the service thereunder. No such contract carrier, unless otherwise provided by this chapter, shall engage in the transportation of passengers or property in intrastate commerce in this state unless the minimum charges for such transportation by said carrier have been published, filed and posted in accordance with the provisions of this chapter. No reduction shall be made in any such charge either directly or by means of any change in any rule, regulation or practice affecting such charge or the value of service thereunder, except after 20 days' notice of the proposed change filed in the aforesaid form and manner; but the commission may, in its discretion and for good cause shown, allow such change upon less notice or modify the requirements of this subsection with respect to posting and filing of such schedules or copies of contracts either in particular instances or by general order applicable to special or peculiar circumstances or conditions. Such notice shall plainly state the change proposed to be made and time when such change will take effect. No such carrier shall demand, charge or collect a less compensation for such transportation than the charges filed in accordance with this subsection, as affected by any rule, regulation or practice so filed or as may be prescribed by the commission from time to time, and it shall be unlawful for any such carrier, by the furnishing of special services, facilities or privileges, or by any other device whatsoever, to charge, accept or receive less than the minimum charges so filed or prescribed; provided, that any such carrier or carriers, or any class or group thereof, may apply to the commission for relief from the provisions of this subsection, and the commission may after hearing grant such relief to such an extent and for such time and in such manner as in its judgment is consistent with public interest and the provisions of this chapter.

(b) Whenever, after hearing upon complaint or its own initiative, the commission finds that any charge of any contract carrier by motor vehicle or any rule, regulation or practice of any such carrier or carriers affecting such charge, or the value of the service thereunder, for the transportation of passengers or property in intrastate commerce in this state is not consistent with the public interest, the commission may prescribe such minimum charge or such rule, regulation or practice as in its judgment may be necessary or desirable in the public interest. Such minimum charge or such rule, regulation or practice so prescribed by the commission shall give no advantage or preference to any such carrier in competition with any common carrier by motor vehicle subject to this chapter, which the commission may find to be undue or inconsistent with public interest, and the commission shall give due consideration to the cost of the service rendered by such carriers and to the effect of such minimum charge or such rules, regulations or practices, upon the movement of traffic by such carriers. All complaints shall state fully the facts complained of and the reasons for such complaint and shall be made under oath.

(c) Whenever there shall be filed with the commission by any such contract carrier any schedule or contract stating a reduced charge directly or by means of any rule, regulation or practice for the transportation of passengers or property in intrastate commerce, the commission is hereby authorized and empowered upon complaint of interested parties or upon its own initiative at once and, if it so orders, without answers or other formal pleading by the interested party, but upon reasonable notice, to enter upon a hearing concerning the lawfulness of such charge or such rule, regulation or practice, and, pending such hearing and the decision thereon, the commission, by filing with such schedule or contract and delivering to the carrier affected thereby a statement in writing of its reasons for such suspension, may suspend the operation of such schedule or contract and defer the use of such charge or such rule, regulation or practice for a period of 90 days, and, if the proceeding has not been concluded and a final order made within such period, the commission may from time to time extend the period of such suspension, but not for a longer period in the aggregate than 180 days beyond the time when it would otherwise go into effect; and after hearing, whether completed before or after the charge or rule, regulation or practice goes into effect, the commission may make such order with reference thereto as would be proper in a proceeding instituted after it had become effective. If the proceeding had not been concluded and an order made within the period of suspension, the proposed change in any charge, rule, regulation or practice shall go into effect at the end of such period.