Post #465: Suntrust Bank, PM peak hour trips: 36, not 381.

Posted on November 21, 2019

This post is sponsored by the Department of Way Too Much Time on My Hands.  It is a followup to Post #460.

I sat in the Suntrust (east) parking lot during yesterday’s afternoon rush hour.  I counted cars going into or out of the lot.  (Each such vehicle movement — either into or out of the lot — is a “trip” in traffic parlance.)  Starting at 4 PM, I counted trips for successive 15-minute intervals, and from those, calculated the trips in the peak hour that occurred between 4 PM and 6 PM.

I counted 36 trips during the peak PM hour.

This should be contrasted with the 381 trips that the Town’s contractor, Kimley-Horn, assumed as part of their estimate of the impact of MAC on Maple Avenue traffic. 

The discrepancy is, in fact, more than a factor of 10 (order-of-magnitude).

Science, my ass.

By overstating the existing traffic by ~350 cars, Kimley-Horn understated the increased traffic from MAC development by ~350 cars.  That’s how the traffic projection methodology works.

Just in passing, I’ll note that this error, alone, for this one building, is on-order-of half as large as the entire estimated impact of MAC on traffic, per the Kimley-Horn study.  In case I need to translate that, it means that, officially and professionally, we still have no clue what MAC zoning is likely to do to Maple Avenue traffic.  With just this one whopping discrepancy, we have simultaneously spent our tax dollars and ensured that anything our contractor gives us in return will be deeply suspect.

And there isn’t just this one discrepancy.  I’m not even going to bring up the minus 1 again (Post #364).  Or the implausibility of the estimates for several other properties.

I’m so depressed by this whole scene that I’m not even going to editorialize about it.  I’m just going to make one comment about real science.

On this particular issue, I think it has reached the point that if I want an credible estimate of this key item, I’m going to have to buy the ITE trip generation manual and do it myself.   The calculation itself really ain’t rocket science.  And that way, I could make the whole thing open-source, instead of a black box, and we could all examine the reasonableness of the results.  We could do what real scientists call a “sensitivity analysis”, that is, test how much the results change if key assumptions are changed.   We could directly test the robustness of the results, so if they really are a shot in the dark, we’d at least be aware of that.

Instead of turning over one rock at a time.

Those of you who have been following this issue don’t need to know any more.  For the rest of you, a brief summary of the story follows.  As well as a full explanation of why this was like shooting fish in a barrel.

Background and results

Let me just briefly tell the story and be done with it. No references, no links.  Relatively few swear words.

  1. The Town of Vienna paid for a “multimodal” study of the Maple Avenue.
  2. They contracted with Kimley-Horn to do the study.
  3. Aside:  Kimley-Horn also does traffic studies for MAC developers.
  4. As part of this study, Kimley-Horn was to estimate how much additional traffic MAC development would bring to Maple Avenue.  We owe Councilmember Noble for insisting on that.
  5. They did this for a “medium-term” projection, based on the assumption that about 20% of the MAC-zone property would be redeveloped.
  6. Taken at face value, their estimate shows that you’d see a roughly one-third increase in cars on Maple in the evening rush hour.
  7. Aside:  That’s not a full build-out of MAC — that’s more-or-less just what the Town was expecting in the next decade or so.
  8. The Town Council and Kimley-Horn have, so far, done their damnedest not to discuss this finding …
  9. At the August work session on this report, Kimley-Horn blew right past the pages showing this finding, and there was zero discussion of it.
  10. Then, hilariously enough, at the most recent (November?) work session on this report, Kimley-Horn refused to discuss this finding — on the grounds that they’d already covered it in August.  (Nope, I’m not making that up.)
  11. Councilman Majdi didn’t sit still for that.  He brought up Kimley-Horn’s estimate for a building he knows well, the Suntrust Bank at the east end of Maple.  (He lives in that neighborhood).
  12. Kimley-Horn assumed that Suntrust, right now, generates 381 trips during the peak hour of afternoon rush hour.
  13. When questioned, both Kimley-Horn and the Town Manager defended the methods used in this report as Science.
  14. Aside:  That number works out to a car moving into or out of the Suntrust parking lot every ten seconds, for a full hour.  It’s not just wrong, it’s ludicrously wrong.  In Science-speak, it “lacks face validity”.
  15. I got so hacked off by the “it’s Science!” defense that I made up my mind to do the  obvious thing and actually count cars.

And so, here we are.

Admittedly, there are some caveats.  My numbers are for just one evening rush hour.  A better estimate would use more days.  Plausibly, I may have mis-counted a bit.  I’m not quite sure what to do with the counts for the parking lot entrance that does not empty onto a public street (they are included here).  I’m not quite sure what to do with cars that just cut through the parking lot and didn’t stop (they are included here).  And so on.

But the results are good enough to make the point, and I’m not going to waste my time to repeat the observation.  The whole process was like watching paint dry, but less fun.  At some point, I found myself rooting for cars to turn into the lot so I’d have something to count.

Plus, the result was completely expected.  For one thing, I have first-hand knowledge from walking past that many times.  For another, Councilman Majdi lives right down the street from this building, and he questioned it.  For yet a third reason, I had already discussed what a magnificent dinosaur that building is, in this section of Post #208.  And, well, just check out the bank’s hours.  Not much traffic after 5 PM.  So getting this result was like shooting fish in a barrel.  Less the mess that would obviously entail.

At that last Town Council work session, I believe Councilmember Majdi got Town Council to agree to use actual traffic counts for existing traffic conditions in any future analysis of MAC impact.  Rather than rely on the ITE ratebook approach.  Given my counts, versus the ITE ratebook as applied by Kimley-Horn, I’d say that was a fairly savvy decision.