Each county or municipality using traffic-control signal monitoring devices shall erect signs on every highway which comprises a part of the state highway system at that point on the highway which intersects the jurisdictional limits of the county or municipality. A sign shall be erected also by such entity on each public road on the approach to the next traffic-control signal for such road when a traffic-control signal monitoring device is monitoring such next signal for such road. Such signs shall be at least 30 inches by 30 inches in measurement and shall warn approaching motorists that traffic-control signal monitoring devices are being employed.