There are a few school zones around here that use partially lit signs. They look much like standard SL signs, except there's a flourescent yellow-green "SCHOOL" banner on top, and the number is done a little differently. In a hole cut in the sign is mounted a hooded light display which, when on, shows "20" in white (or possibly yellow-green) on black. It looks like the number is made of LEDs, but given the age of the signs, it might be incandescent light shining through holes in a black mask. Sometimes there are also yellow lights above and below the sign which flash when the speed zone is in effect.