I have one manual that states it is a speedwarning lamp but I don't quite understand the purpose of the reflector. I do remember in some countries in SE Asia you had devices to prevent people from speeding. In Malaysia, if I remember well, all commercial vehicles had an orange lamp on top. If they we're going about the maximum speed the lamp would start to blink and if they were going too fast for too long the lamp would light up constantly drawing the attention of the cops alongside the road.
Although I've never witnessed it myself, I've heard of another country (could be Japan that I've never travelled) where horns would sound automatically and that they could only be switched of by a officer (with a special key) who ofcourse would write out a ticket at the occasion.