When the construction of sections of the parkway road parallel to existing highways will result in unnecessary expense and the secretary in his discretion does use existing or projected sections of streets or highways, bridges, parks, or other areas as connecting links between other sections of this parkway, and federal funds are expended for parkway purposes thereon under the provisions of such act, no parkway area credits need be allowed the state therefor. In the event it is determined that such connecting links of highways should provide for other than passenger car traffic, a cooperative agreement with respect to such use and the reconstruction, maintenance and control thereof may be entered into by the secretary with the state transportation department or local public authorities concerned. Alternate routes through or around urban areas, along scenic country or connecting historic sites, may be provided or required for acceptance by the secretary for parkway purposes.