Mounting Hardware

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Although the requirements for signal placement are well defined in the MUTCD, the actual techniques used to mount traffic signals vary widely across the country. Signals can be mounted at the side of the road, on the tops of pedestals, on the sides of poles, or over the roadway suspended from span wire cables or mounted on a rigid cantilevered mast arm structure.

Pedestal and Pole Mounting of Signals

Pedestals and poles can be used to mount both vehicular and pedestrian signals, although the technique is becoming less popular for vehicle signals because of the improved visibility that can be obtained by an overhead mounting. Poles are vertical structures designed to mount vehicular signals at a height of 8 to 15 ft above the roadway, or pedestrian signals at a height about 7 to 10 ft above the sidewalk. They are usually made of steel or aluminum pipe. Mounting of the signals to the top of these supports typically is done with a cast aluminum slip fitter cap, an adapter that fastens to the top of the support with setscrews, and provides one or more threaded hubs for standard signal mounting hardware.

Signals can also be mounted on the sides of utility, lighting, or signal support poles, at the same elevations used for pole mounting,

Span-Wire Mounting Techniques

Suspending signal heads over the roadway from cables stretched between two poles has long been an appropriate signal mounting technique in some jurisdictions. Span-wire mounting provides a convenient attachment location for over-the-roadway signal displays, typically with a much lower installation expense than a mast arm or comparable structural mounting configuration. However, span-wire installations do not provide the mounting rigidity available with other structures, and may also present an objectionable unaesthetic appearance.

Span-wire installations include two key components- the supporting poles, called strain poles, and the cable assembly. Strain poles are designed to withstand the great forces placed on the cable attachment point by the weights of the signal heads on the long span cable. Poles include tubular steel structures, either directly embedded or mounted, through an anchor base assembly, to embedded anchor bolts; timber poles, which typically need guy wires to help resist the bending movement of the pole, and embedded pre-cast concrete poles.

Supporting cables are made of high-strength galvanized wire cable. Some jurisdictions use only one cable to support all heads, others use two or three cables to support all heads, or even a cable dedicated to each head. Electrical cables to the signal heads are secured to the supporting cables, using lashing rods, cable rings, or other similar devices.

The installations provide considerable flexibility in head placement. Although configurations are almost unlimited, the two most common designs are the diagonal span, in which two poles support a cable running diagonally across the intersection, and the box span, in which poles, located on each corner of the intersection, are connected together by a cable assembly to form a "box". When a box span is desired, but the poles must be set back from the intersection for recovery purposes, a modified (or drop) box span can be used. A cable box, typically formed along the extensions of the four approach curb lines, is suspended by four diagonal cables leading back to the strain poles.

Mast Arm Mounting Techniques

Mast arm assemblies are cantilevered structures designed to locate signal heads over the roadway. The arm shapes. Arm assemblies are available in a number of different designs. A mono tube arm assembly provides a clean, although sometimes overwhelming, appearance. a truss type of arm uses two cantilevered members, each of which is smaller than would be used in a mono tube design. A trombone arm is a variation of the truss; the arms structural members are parallel, and signal heads are usually mounted horizontally between the two members.

Mast arm structures provide substantial rigidity for signal mountings, but are limited in their adaptability to unusual intersections. Some designs permit the temporary rotation of the arm out of it's normal position, a benefit where over height loads are occasionally encountered.

Most mast arm assemblies are used in a "box" configuration, where a pole and arm are located on each corner of the intersection. Some jurisdictions, like New York, use a diagonal design, with the two poles and arms pointing at each other diagonally across the intersection. Signal heads can either be rigidly mounted or can be free swinging below the arm. Rigidly mounted signals (particularly ones with back plates) add considerable wind loading to the arm, an important concern in structure design. Most poles are rated to withstand 95 MPH sustained wind force, much higher than that of other methods.

Examples of extreme mounting!  This is called "sewer pipe" mounting by locals. Click to enlarge photos.

Example of gantry mounting.

Signal Mounting Hardware

Signal mounting hardware, like the signal head itself, must be of a rugged, durable design. Various types of hardware are available to adapt standard signal heads to the different mounting techniques used across the United States.

Mounting Hardware for span-wire installations varies widely by jurisdiction. A saddle clamp, or clevis, which rests on the supporting cable, is commonly used. Signal hangers are designed to attach to the clevis with a pin, and provide for a weather head cable entrance into the signal head.

Disconnect hangers are terminal boxes made integral with the signal hanger. The disconnect hanger assembly is permanently mounted on the span wire. A terminal strip for attaching the signal feed wires is provided in the box, and a multi-pin plug and socket allows for easy replacement of the signal head itself.

Mounting brackets for mast arm mounted signals provide two options-- a free-swinging, below-the-arm mounting, or a rigid mounting. Free-swinging mountings are basically the same as span-wire fittings. Rigid mountings come in a number of different configurations. An elevator plumbizer is a tubular pipe extension, flattened out to attach, in line, between two sections of the signal head. It can either be used at the end of a mast arm, with the bracket coming out of the side of the signal head, or at intermediate points along the arm, with the bracket extending directly behind the signal head. As the mounting creates a gap between sections, the technique frequently is used with custom back plates to minimize the visual discontinuity.

A number of different adjustable brackets, some of which are patented designs, are available for mast-arm mounting applications. These brackets, which typically support the signal head at both ends, permit full directional adjustment of the signal on the arm. Special brackets are necessary for programmed visibility signals, because of the greater rear clearance required.

 

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