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Wear: How a 10-foot-high sound wall can hide behind a 6-foot fence


Frances Allen, a retired teacher, has been pushing for sound walls along MoPac for 25 years.

Allen, who lives near Hancock Drive, had been promised a 10-foot wall as part of the MoPac toll project.

But that measurement came from the road surface, not her yard, so an existing 6-foot fence mostly blocks it.

I can report to you that finally, after a quarter century, Frances Allen, the MoPac sound wall woman, has her sound barrier.

And I always assumed that when this day came — I’ve written several stories about Allen, her quest for relief from MoPac traffic noise and the delays in MoPac sound wall construction — it would be that rare time when a journalist gets to tell you an unalloyed, good-news-feel-good story. Unfortunately, that is not the case.

Allen, who lives in a tidy Central Austin home backing up to MoPac Boulevard’s east side about a half-mile south of Hancock Drive, woke up March 4 to find that the promised 10-foot concrete wall (being built as part of the MoPac toll project) was finally in place. And that she could barely see that 10-foot wall over the top of her existing 6-foot-high wooden fence.

Eh, what?

“Only about 3 to 6 inches are sticking out” above the wood fence, Allen told me when I called. Because, you see, the Central Texas Regional Mobility Authority and its contractor measured and built the 10-foot wall from the level of the highway, not from the level of Allen’s backyard. And the grass and ivy of her modest yard is about 4 feet above MoPac’s elevation. So for the wall to appear to be 10 feet high from her patio, it actually would have had to be 14 feet high from its base.

“Wow, wow, wow,” Allen said. “It really made me sick at heart when I realized what had happened. Anyway, it’s there to stay, I guess.”

Allen said several of her neighbors along Westfield Drive are equally distraught over the stubbier-than-they-expected wall. One man a few houses away, she said, “is so mad he can’t even talk about it.”

RELATED: Explaining the delay, and repair, of MoPac sound walls

I should say, right away, that the mobility authority told me that there has been no design or construction mistake. When the Texas Department of Transportation and the authority studied the MoPac traffic noise situation as part of an environmental study several years ago, and determined where walls were required under federal law, the listed wall heights were always tied to the highway pavement elevation.

The yard heights alongside the road, as they point out, vary even within a given neighborhood. Allen’s daughter, for instance, lives a few houses north of her at a somewhat lower elevation and thus the new wall looks taller from the vantage point of her backyard, Allen said.

“The main goal of the walls is to block the line of sight from the tires and wherever people are,” said Steve Pustelnyk, the mobility authority’s director of community involvement, who has gotten to know Allen and other MoPac neighbors very well over the past several years.

Almost all of the sound coming from a passing vehicle is generated by the tires rolling over the pavement, said Lloyd Chance, the authority’s construction manager.

So why would Allen and others have rolled along all these years, through innumerable meetings with various highway folks, and never have comprehended that the wall height would be measured that way? I’ve even put that figure in previous columns about Allen, who bought her home in 1993, talking about how the wall would loom over her wooden fence.

“That was a communication error, and I don’t really know how to explain it,” Pustelnyk said.

As for the sound suppression, which was the point of all this, it’s hard to tell what if anything has been accomplished.

I visited Allen in late 2016 and sat in that backyard, listening to the whoosh and thrum of passing cars and trucks with her. We had to talk at somewhat raised levels to each other, but it wasn’t exactly a din.

I drove back out there last week, post-wall, and we again went out back. As it happened, just as we exited her back door a freight train rumbled by. For that minute or so — and more than two dozen freight trains come by each day in MoPac’s median — Allen and I basically had to yell at each other. Or wait.

“When the train engines are going by, we just stop talking,” Allen said after things had quieted a bit.

You can see the tops of the train cars over her fence (and wall) as they pass. And when a big truck carrying new cars went by a few minutes later, we could see part of it as well. Allen had been counting on a taller wall to block those visuals.

Pustelnyk and Chance told me about the applicable federal rules, and they pointed me to the section of that environmental impact study where sound and walls are discussed for about 25 pages.

The simplified version: Federal law requires some sort of sound mitigation during an urban road project, or at least the offer of such to the property owners directly alongside the road, when the average sound level in a residential area exceeds 66 decibels. And a wall, if that is what is found to be “feasible and reasonable,” must cut the sound level on average by 5 to 7 decibels.

Frances Allen’s long quest for MoPac sound walls

In Allen’s neighborhood, the average sound level before construction of the toll project was found to be 78 decibels at one sound receiver and 79 at another. And the engineers estimated it would increase to 81 decibels by 2030 if the project were built. And that a 10-foot wall — from highway level — would provide that required drop of at least 5 decibels.

The decibel scale, you need to know, is logarithmic rather than linear. The upshot is that the sound magnitude goes up in multiples of 10 for every 10-decibel increase. So 70 decibels is 10 times louder than 60 decibels, and 80 decibels is 100 times louder than 60 decibels. Every decibel really matters, in other words.

(Don’t worry, there won’t be a test after this column.)

The mobility authority, as part of a contract to be awarded later this spring, already intended to replace all the 6-foot wooden fences along MoPac between the lake and Far West Boulevard, built originally by TxDOT, because many of them were removed for construction or are in lousy shape. Allen has asked the authority to build her a 10-foot wood fence instead. Perhaps that would knock off a decibel or three.

The authority is pondering that request, Pustelnyk said.

Allen, given her somewhat high-profile advocacy for the walls, has taken some knocks along the way. The gist: You knew what you were getting into when you bought a house along a highway.

“People through the years would say, ‘Why don’t you just move?’” said Allen, a retired teacher who turns 83 the day this column will run in print. “You know what? I should have taken their advice.”

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