The department shall take over and accept as a part of the state highway secondary system the roads remaining in the various county road systems which have been maintained by the respective counties, or so much mileage thereof as the availability of funds for construction of secondary state highways in a county may justify; provided, that municipal streets which are extensions of state highways may be added to the state highway secondary system in lieu of an equal mileage of county roads. The roads to be placed in the state highway system hereunder shall be selected by the department. Maintenance jurisdiction by the department of roads added to the state highway secondary system pursuant to the provisions of this section shall not commence until construction to state highway standards shall have started.