Post #358: A one-third increase in Maple Avenue rush hour traffic.

Posted on August 21, 2019

The Kimley-Horn analysis presented at the 8/19/2019 Town Council work session estimated that even a modest amount of MAC development on Maple would result in (by my calculation) a one-third increase in peak rush hour traffic on Maple. 

I believe that’s the single most important fact to come out of the 8/19/2019 Town Council work session and meeting.  And yet, the contractor (Kimley-Horn) and the combined TC/PC/TSC blew past that so fast, nobody even bothered to discuss it.  So, since they didn’t talk about it, let me walk you through it.  With my usual citations as to sources and methods.

But first, I have to acknowledge a tremendous debt to Councilman Noble, who, as I recall, had to do everything short of pound his fist on the table to get this analysis included in this study.  I’m going to pile on the sarcasm later, but here, I am completely serious. This analysis was not part of the original proposal for this report.  But this is one of the numbers we must have, in order to have any rational discussion of what’s best for the Town of Vienna.  And if he hadn’t persisted, this would never have seen the light of day.  So, we, the citizens of the Town, owe Councilman Noble a debt of gratitude for prodding the Town into taking this step in the direction of having a rational, fact-based discussion of the future of Vienna under MAC.

Rational means discussing the pros and cons of your actions.  But the Town barred any discussion of traffic during the development of MAC.  So this action rights a long-standing error, and starts us down a path toward a rational discussion of the issues.

Detail follows.


First you need a tiny bit of context in order to understand what the Kimley-Horn report showed.   That context is this:  How many cars pass down Maple Avenue, now, at the peak rush hour?  Answer:  About 2366 cars.

To be clear, that’s 2366 cars, passing a point on Maple, during that peak hour.  To see how I got that, turn to Virginia Department of Transportation traffic count data.  Download the 2018 Town of Vienna data from this link (.pdf).  Scroll to page 7.  Here it is with full detail.

Here it is again, simplified to just the key numbers.  (Sorry if you have to squint, this is how it looks in the .pdf from VDOT).

This is the street segment from Nutley to Follin.  At the far right is average weekday traffic of 33,000 vehicles per day.  Moving to the left is the “K factor” of 0.072, which is the fraction of daily traffic that was observed during the peak traffic hour.  (In this case we can tell that this number was an actual measurement by VDOT.)  Multiply the two together to arrive at (33000 x 0.072 =) 2366 vehicles during the peak rush hour, as measured by VDOT.

(The scientifically-trained among you will laugh at four significant digits for this number, but let’s just continue.)

OK, now you’re ready to understand the Kimley-Horn analysis.  First, get a copy of the Kimley-Horn 8/19/2019 presentation from the Town’s website, at this link (.pdf).  Turn to slide 10.  Without (yet) commenting on the quality of those numbers, let me just take them at face value. The key number is in boldface near the bottom:  +758 PM peak hour trips.

So, how much does rush hour traffic increase, under this scenario?  Well, that’s (758/2355 =) 32%. 

Or, in round numbers, a one-third increase in rush hour traffic.

A short break for some sarcasm.

Well, who could possibly have guessed that?  Who would have thought that concentrating a bunch of high-density housing projects on the single most congested street in Northern Virginia* might result in — more congestion?  Completely unexpected.

* OK, yeah, I made that up, about the single most congested street.  But if there were such an award, don’t we all think that Maple would be in the running for it?

Proper discussion.

First, the most important thing to realize is that this is only a partial build-out of all the available MAC property. The entire MAC zone contains 106 acres, more or less.  At the time MAC was passed, both Town Council and Town staff were cited as guessing that more than 60% of that acreage would eventually be redeveloped under MAC.

So, by eye, if you combined all the colored blocks on the picture above, what fraction of Maple was assumed to be redeveloped, for the Kimley-Horn estimate?  Knowing what I know about those lots, my guess is 20%.  So, this estimate says that if 20% of the MAC zone gets redeveloped under MAC, you’ll see a one-third increase in peak rush hour traffic.

By extrapolation, then, what the Town originally envisioned as a full build-out of MAC — 60%-ish of the entire MAC zone — would result in a doubling of peak rush-hour traffic.

You know, love MAC or hate MAC, that’s something we really need to think about.  Shame on those who forbade any discussion of traffic as MAC was being developed.  You did your Town no favors by doing that.  And, again, kudos to Councilman Noble for finally getting some numbers on the table.

Second, are there any obvious shortcomings of those Kimley-Horn numbers.  Yes, one leaps off the page:  Giant Food shopping center.  Look at the largest teal box on the picture above.  That’s the Giant Food shopping center.  They are claiming that redevelopment of that lot — all 10 acres of it — would result in just 65 additional rush-hour trips.

Hahahah.  No.  I would love to see the contortions they had to go through to get that number.  If they keep the retail — which you kind of hope they do, if you buy groceries in Vienna — then what you ought to see is the rush-hour trips generated by all the new housing.  So far, these MAC projects are averaging about 100 new Town residents per acre.  That’s a 10-acre lot.  So what you ought to see there is the Maple Avenue rush hour trips generated by maybe 1000 new persons, call that maybe 600 new adults, living on Maple.  I don’t think that just one-tenth of them will have jobs that require them to commute.

So, from a flat-footed common-sense perspective, it sure looks to me like they shaved that Giant shopping center number down.  Quite a lot.  There’s no telling whether that was a one-off, or whether all those projections were systematically shaved down.

Finally, can we “triangulate” this in any way?  That is, compare it to some independent estimate, done by some other method, to assure us that the Kimley-Horn estimate is in the ballpark?

Why do that?  There’s a lot of potential for slack in these traffic analyses.  A lot of potential for judgment calls.  And you’d like to see somebody else’s analysis to be sure there is no large systematic bias in the numbers.

I did my own crude estimate about a year ago, for total traffic (not rush hour), simply by extrapolating the 444 Maple West projections.  That’s my post titled “Traffic and the Ultimate Impact of MAC Zoning“, posted 7/26/2018.  Let me dig that up and compare to the Kimley-Horn estimate.

This is my crude estimate, from one year ago, impact on total (not rush hour) trips on Maple:

A year ago, I crudely estimated that a full build-out of MAC would increase total trips on Maple by 118%.  Extrapolating the Kimley-Horn number to a full build out of Maple implied an increase in total trips on Maple of 100%As these things go, that’s a bullseye.  My crude extrapolation and the Kimley-Horn numbers are definitely in the same ballpark.

I should probably mention other technical details and caveats that might materially matter, such as the treatment of “bypass” trips.  I should equivocate over the Chick-fil-A-car-wash number, because some fraction of those might never make it literally onto the segment of Maple between Nutley and Follin.  I should probably ask about the fraction of trips that might not make it to Maple at all, because I can’t know exactly what the Kimley-Horn analysis did.  (I assume that because this is about Maple, those are estimated Maple trips, but I can’t be sure of that.)  But let me stop here.  I think this is good enough for now.

I’m going end by repeating what I said more than a year ago, in the post cited just above.  If it sounds kind of strident, well, maybe in hindsight that was perfectly justified.

“The bottom line here is that under any reasonable set of assumptions, this is a significant and material issue, and the Town needs to stop avoiding it.  They need to look to the future, see what kind of Maple Avenue they are creating, and act now to avoid the worst impacts.  I can also understand why a pro-growth Town Council would not bother to do this basic calculation.  There is no way to paint a pretty picture here.  But the fact is, if the Town is bent on remaking Maple under MAC, they need to do this calculation properly and restructure MAC to give us something tolerable on Maple.  As it stands, there appear to be no long-term Town estimates of the ultimate impact of MAC.”

But now there are some estimates.  That’s progress.