Cities and counties shall have the power to establish school zones and provide for the stopping of all motor vehicles approaching the school zones, when movable stop signs have been placed in the streets in the cities and highways in counties at the limits of the zones, notwithstanding the provisions of any statute to the contrary. All traffic-control devices provided for school zones shall conform to specifications included in the manual of traffic-control devices adopted by the department, except the provision prohibiting the use of portable or part-time stop signs.