Mo. Rev. Stat. § 307.122

No motor vehicle or trailer shall be operated on a public highway of this state while equipped with any device which emits an electronic message directed to the front, side or rear of the exterior of the vehicle or trailer. For the purposes of this section, the term "message" shall include words, phrases, sentences, numbers and other symbols or combinations thereof. This section shall not prohibit the lawful use of a lamp which illuminates the rear registration marker, as authorized pursuant to section 307.075, or the use of a route indicator on a bus or other public transportation vehicle, or messages that display proper names of firms or corporations. Violations of this section shall be a class C misdemeanor.