Driveway approaches shall be a minimum of fourteen feet wide at the curb or curb line, the subgrade thereof shall consist of granulated slag, gravel or stone, the minimum thickness of which shall be three inches, and the surface of which shall be of rigid construction not less than six inches thick. The installation of such driveway approaches shall be under the control and direction of the Director of Public Service, who may require that the subgrade thereof shall consist of granulated slag, gravel or stone more than three inches thick and who may also specify the character of the surface thereof which may be in excess of six inches.