The evolution of adjustable traffic signals in the early 20th Century can be very roughly classified into three basic design patterns. Many, though not all, of the major signal manufacturers’ evolved through the following three design patterns.
Solid Cast Signals:
To make signals easier to configure and repair, each lamp module is individually cast with open ends, and affixed by an access door on the front. The modules can be stacked together in any number, as required. The stack is enclosed by end plates at top and bottom and held together by tie rods through the length of the signal head.
Taking the best aspects of earlier designs, solid cast individual lamp sections with a hinged access door on the front are simply bolted together as needed to create a traffic signal face. The end plates and tie rods are gone, although they were supplied by some manufacturers when antiquated local specs required them (e.g., “Finned Marbelites”).