The evolution of “adjustable” (i.e. single-face) 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’ adjustable signals evolved through the following design patterns.
Solid Cast Signals:
Separate castings were fashioned for 1-, 2- and 3-lamp configurations. Engineers desiring, say, a 4-lamp inline signal configuration would need to stack two 2-lamp housings, or a 3-lamp and 1-lamp housing.
Beginning early on, tie-rodded designs were adopted by some manufacturers. Both Crouse Hinds and American Gas Accumulator were using tie rods by the late 1920s. To make signals easier to configure and repair, individual lamp segments with open ends were affixed with an access door on the front. These modular segments could be stacked together in any number, as required. The stack was enclosed by end plates at top and bottom and held together by two or three tie rods through the length of the signal head. The signal probably most well-known by collectors is the Crouse Hinds Type D/DT “Art Deco”, which used a tie-rodded design from the ’30s well into the ’50s.
Taking the best aspects of earlier designs, individual enclosed lamp segments with a hinged access door on the front were simply bolted together as needed to create a traffic signal face. General Electric may have pioneered this design with their sectional signal introduced in 1937. The end plates and tie rods were gone, although Marbelite still supplied them for a few years in places where antiquated local specs required them.