No privilege or franchise granted after May first, one thousand nine hundred and eleven, to any public utility by a political subdivision of this state shall be valid until approved by the board. Such approval shall be given when, after hearing, the board determines that the privilege or franchise is necessary and proper for the public convenience and properly conserves the public interests. In granting its approval the board may impose such conditions as to construction, equipment, maintenance, service or operation as the public convenience and interests may reasonably require.

Nothing herein shall require the approval by the board of any consent of a municipality or board of boulevard commissioners for the operation of any autobus in effect March twenty-fourth, one thousand nine hundred and twenty-six, which did not then require such approval to be valid.