Directional signs showing distance and direction to points of importance may be placed at all crossings and intersections of primary and secondary state highways. The department may place such directional signs as it deems necessary upon any city streets designated by it as forming a part of the route of any primary or secondary state highway through any incorporated city or town. Caution and warning signs or signals shall be placed wherever practicable on all primary and secondary state highways in a manner provided by law. Stop signs shall be placed, erected, and maintained by the department as follows: Upon all county roads at the point of intersection with any arterial primary or secondary state highway; upon all primary and secondary state highways at the point of intersection with any county road that has been designated by the department as an arterial having preference over the traffic on the state highway; and upon at least one state highway at the intersection of two state highways.