Parkway areas as acquired by this state and scenic, landscape, sightly or safety easements on additional areas, shall be in accordance with preliminary development and property maps submitted to the state by the secretary. State agencies and interstate or intrastate utilities having fiscal or other interests in any such properties may join in or execute releases for such parkway areas or easements. Parkway area acquisition shall be undertaken in units of sufficient length to justify placing such units under contract for construction of the parkway road as soon as acquired by the state and accepted by the secretary.