Every turn signal used to give a signal of intention to turn a vehicle shall be so constructed and so installed as to give a signal plainly visible in clear weather and under normal traffic conditions from a distance of at least 100 feet to the rear and 100 feet to the front of the vehicle. No front turn signal, however, shall be required on vehicles manufactured before January 1, 1943. The Superintendent may promulgate regulations not inconsistent with this section and § 46.2-1038 governing the construction, location, and operation of turn signals and the color of lights which may be used in any such signal device. Nothing contained herein, however, shall prohibit the requiring of turn signals on any vehicle whose driver is prevented by any reason from giving the hand and arm signal required in § 46.2-849.