A support tube assembly for use in mounting traffic signals and other traffic control devices. The tube has a standard 1.9 inch outer diameter with a wider three-quarter inch access slot and a larger internal volume to accommodate five-eighth inch conductors. The support tube assembly may include a secondary mounting assembly for attaching a second traffic control device. The secondary mounting assembly includes a mounting strip that fits inside the access slot and a mounting plate that attaches to the strip. Each edge of the access slot has a contoured profile that mates with the corresponding side on the mounting strip so, when the secondary mounting assembly is attached to the tube, the edges of the access slot are gripped between the back of the mounting plate and the inner mounting strip. A cut-to-fit vinyl closure strip is included to enclose the exposed sections of the access slot.