The governing body of any municipality may enter into a written agreement with the department for the maintenance of such additional width by the department, and from time to time in accordance with such agreement shall appropriate and pay into the state highway fund such sums of money as may be agreed upon. Nothing herein contained may be construed to prevent any such municipality from maintaining such additional width at its own expense subject to the written approval of the department.
N.D.C.C. § 24-01-11