All streets shall be improved at edge of the paved roadway with portland concrete curbs. Such curbs shall be similar in design and construction to concrete curbs adopted as standard for the city unless special conditions warrant the waiver of this requirement by the planning authority. Exception: Curb on access and rural collector streets are not required in a S-1 suburban zone. In lieu of curbs, roadside ditches and culverts shall be constructed. This exception shall not apply on arterial routes with the S-1 suburban zone.