In addition to equipment required under section 1283 of this title, any school bus as described in subdivision 4(34) of this title may be equipped with a stop arm sign with flashing red lights, of a type approved by the Commissioner, securely mounted below the left front window on the vehicle in an area approved by the Commissioner and must be visible to the driver when extended. Such sign shall not be activated unless the vehicle has stopped on the highway for the purpose of receiving or discharging public or private schoolchildren. At no time shall the sign be extended when the vehicle is in motion. (Added 1969, No. 49, § 1; amended 1975, No. 149 (Adj. Sess.), § 5; 1977, No. 102, § 5, eff. Sept. 1, 1977.)