Where any public highway being constructed, improved, or maintained by any separate road district shall run or extend into or through any incorporated municipality within the said district, such highway may be constructed, improved, and maintained the same within as without the corporate limits of said municipality. With the consent of and in conjunction with the road commissioners, the said municipality may, out of its own funds, add such street crossings or make improvements, such as grading, culverts, graveling, paving, or other improvements, as it may desire; the mayor and board of aldermen shall, by an order spread upon their minutes, consent for the work to be done by the commissioners. This section shall not be construed as any limitation upon those sections of this title in reference to state highways.