Episode 41 – Mast Arms, Strain Poles, Span Wires and other Signal Structures

Topics: Mast Arms, Strain Poles, Span Wires and other Signal Structures



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Hello and welcome to another edition of talking traffic. My name is Bill Ruhsam and I host this podcast and its sister website, talking traffic .org. Today is Monday February 13, 2012. This is episode 41 and it is about mast arms, strain poles, span wires and other signal structures.
Have you ever thought much about how a traffic signal is erected and supported? I mean, we’ve got these green/yellow/red lights hanging in the air, being powered by electricity, but have you thought about what’s holding them up? I have, but then, that’s part of my job.
There are several considerations when choosing the infrastructure of your signal support. Firstly, and some might say most importantly, there is cost. Then there is size, and durability, maintainability, aesthetics, local weather conditions, and also the policy of the agency that is building the signal. This all gets roiled around in the cauldron of decision making which spits out an answer. These answers (at least in America) generally fall into two categories: Mast Arms or Span Wires.
A Mast Arm is a metal tube extending out over the intersection which is bolted onto a pole that sticks out of the ground. Generally, they are made of steel. In fact, now that I mention it, I don’t think I know any mast arm and pole assembly that is not steel. If you’ve ever seen one, let me know.
Span Wire assemblies start with poles sticking out of the ground and then steel wire is strung between them in a box, or other shape to support the signal heads. The poles that are used can be wooden, concrete, or steel. You will often see these sorts of poles sharing their span wire attachments with overhead utility lines like electrical service and cable TV. It makes sense to keep the number of poles in a particular location to the minimum number possible.
The signal lights ( we call them signal heads, and just to be even more pedantic, each individual green red or yellow light is called a signal section)… the signal lights are hung from the span wire or the mast arm along with any signs that might need to be up there. The wires that power the signals are routed through the mast arms and come out of holes at each signal location, or they’re fastened directly to the span wire and run along to each signal head.
The pros and cons of these two types of signal installations: As I said earlier, there are cost considerations, span wire will be cheaper and faster to install than mast arms. From a durability mast arms will do better than span wire, generally. Aesthetically, most people agree that mast arm installations look better than span wire, both from a professional perspective, and because you don’t have a bunch of wires that look ugly. On the maintenance side of things, it’s probably almost a wash because while you’ll have to do more maintenance to a span wire installation, if you need to move signal heads around for some reason, span wire is much easier. That leads us to local weather and local policy.
Policies are very different from place to place. Some agencies prefer to install mast arms because of the aesthesis and durability. Others prefer the cost and time savings over other factors. One thing that most look at, and if you’re in a southern coastal state in the U.S., you probably already know this, is wind speed. Span wire signals will blow around in high winds like nobody’s business and that can lead to damage to the signal. High wind-prone areas tend to go with mast arms as a matter of policy because they are sturdier. They also tend to hang their signal heads in a horizontal position, rather than vertically, to avoid excessive swaying in the wind.
There are other, more esoteric solutions to this problem of how to hang signals. If an intersection is exceptionally large, you might see what’s called a monotube installation, which is a mast arm that extends from both sides of the intersection, making a bridge. Also, there are signal span structures which are big and ugly and look like box girders on steroids with teeny tiny signals mounted on them. At least, that’s what *I* think.
Another preference for the mast arms revolves around using video detection technology rather than in-pavement loops. If you listen to Episode number 8 of talking traffic, you’ll learn all about those, but the short story is that you can’t put a video camera on a span wire. You have to mount it on something fixed and firm, either the pole off on the side of the road, or on the mast arm. Mounting it on the mast arm is the ideal situation because you have the best “look” at the approaching traffic and don’t have to worry about a large truck in the through lane triggering the left turn signal, simply because the truck breaks the “view” of the video detector.
So, there are two basic types of signal installations: mast arms and span wire. They both have their plusses, minuses and tradeoffs. There is no clear winner.
I hope you enjoyed this episode of talking traffic. Next time I’ll be back with an interview with Steve Williams, the Georgia Road Geek. He and I will talk about what is a Road Geek and his website and online radio program.
If you have any comments about today’s episode, please leave them on the show notes. or you can send an email to bill at talking traffic.org. I’d be delighted to hear from you.
This episode is released under a creative commons attribution, non commercial, share alike license. Send it to your friends and use it for your book reports, but please give me the credit when you do and don’t make any money from it. Or I’ll send out my legion of span-wire gnomes. They’re the ones that clean the signal heads at night.
The music you hear is by five star fall and can be found at magnatune.com
Until next time, have a great week.

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2 Responses to Episode 41 – Mast Arms, Strain Poles, Span Wires and other Signal Structures

  1. alyson whitted says:

    I’m part of a group at my high school that works on designing sustainable bridges. My question is Can you go in to more detail on mast arms and span wires. I would like to know how they apply to bridges. sort of like a “Mast arms and Span wires for Dummies”.

  2. Bill Ruhsam says:

    Alyson. Thanks for listening.

    I confess that I personally can’t go into much more detail. I’m a traffic engineering specialist, rather than a structural engineer. However, I will keep this in mind when I’m looking for people to interview.

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