Mo. Rev. Stat. § 300.065: Emergency and experimental regulations.

1. The chief of police by and with the approval of the city traffic engineer is hereby empowered to make regulations necessary to make effective the provisions of the traffic ordinances of the city and to make and enforce temporary or experimental regulations to cover emergencies or special conditions. No such temporary or experimental regulation shall remain in effect for more than ninety days.

2. The city traffic engineer may test traffic control devices under actual conditions of traffic.