Portland cement concrete sidewalks having a minimum width of four feet and having a minimum thickness of four inches shall be installed on both sides of all new streets, provided, however, that the Planning Commission may waive this requirement and allow sidewalks along only one side of any new street where in its sole and absolute discretion it determines that sidewalks on both sides of such new street are not necessary. Beyond the corporation limits, in areas where the predominant lot width is 100 feet or more, the Planning Commission at its discretion may waive the requirement for any sidewalks.