After July first, one thousand nine hundred and twenty-eight, the cost and expense of maintenance, construction, and improvement of all the highways herein described, shall be paid out of moneys appropriated to the State Highway Department for the maintenance, reconstruction, or improvement of State highways: Provided, however, That any route contained in this act, which leads into a new industrial community, may be constructed, reconstructed, or improved prior to July first, one thousand nine hundred and twenty-eight, if the State Highway Department, in its discretion, considers such improvement necessary.