(a) The commissioner may, at any time after notice and opportunity to be heard and for reasonable cause, suspend, revoke, alter, or amend any certificate issued under this article, under the "Motor Carrier Act of 1929," under the "Motor Carrier Act of 1931," or under prior law, if it shall be made to appear that the holder of the certificate has willfully violated or refused to observe any of the lawful and reasonable orders, rules, or regulations prescribed by the commissioner or any of the provisions of this article or any other law of this state regulating or taxing motor vehicles, or both, or if in the opinion of the commissioner the holder of the certificate is not furnishing adequate service, or if the continuance of said certificate in its original form is incompatible with the public interest.

(b) If and when the commissioner shall undertake to revoke or modify any certificate on account of the public interest on the ground that the traffic conditions are such as not to justify the number of motor carriers which have been granted certificates over the route or routes in question, the preference shall be given to certificates in order of the time of their issuance, so that those which have been issued later in time shall, other things being equal, be canceled rather than those issued earlier in time.