Where higher standards have not been established as specified in Section 20.32.010; all streets and arterials shall be platted according to the following minimum, except that in difficult-to-develop land surrounded by developed areas a lesser standard approved by resolution of the city council may be utilized upon recommendation of the community development and engineering departments. A planned community or planned unit development; if designed with a comprehensive circulation and parking system, including separate pedestrian ways; may justify modification of standards. Higher standards may be required; in the discretion of the city engineer; where streets are to serve commercial or industrial property or where probable traffic conditions warrant. Private streets shall not be permitted except as set out in Section 20.28.020.