Post #361: Yet a third cut at MAC-generated traffic

Posted on August 23, 2019

You might wonder how I managed to pick up on the Kimley-Horn traffic result  Post #358.  I mean, they did their best to flash that across the screen.  They provided more-or-less zero discussion.  None of the meeting participants said boo about it.

In fact, until I actually got my hands on the presentation, I thought I must have misread it, as you can see from the struck-out section of Post #357.  How could a result like that have been presented with no discussion?

But I remembered it because I was prepared to see it.   Or something very much like it.  Why?  Because the answer to the question “will MAC development add materially to Maple Avenue traffic” is pretty obviously “yes”.  I had already convinced myself of that more than a year ago.  Different methods will lead to different estimates, surely.  But even now, no matter how I slice it, that’s the answer I still come up with.

In this post I present yet a different calculation to suggest that this is plausible.  Just a rough cut, no real work involved.  I’m going to gin up a crude guess for the number of peak hour Maple Avenue trips you might expect, just from commuters living in all that new MAC housing.

In this calculation I’m ignoring all traffic to the retail space.  I’m pretty sure that will understate total new traffic, but for now, just put that aside.  I’m also going to ignore (e.g.) high-school kids driving to school, or parents driving kids to school, pre-school, or day care.  I know that will understate new traffic.  And so on.  Just ignore everything but commuters.

Here’s my pro-forma calculation. New peak hour commuting trips on Maple, from these new MAC developments, would be calculated as:

  • MAC acres x
  • Average persons per MAC acre x
  • % adults x
  • % of adults who work x
  • % of workers who drive to work x
  • % who will drive some of that on Maple
  • % of those trips occurring during the peak hour.

I think the flow of logic is so obvious that I don’t need to explain it, except maybe the last line.  Traffic data and studies typically cite “peak hour” traffic (because, among other things, “rush hour” is not a well-defined term, and “rush hour” varies from place to place).  To compare with anything done by traffic professionals, or with VDOT traffic data, I have to take (say) total morning commutes, and guess what fraction of those will occur during the peak hour of the morning rush.

Let me simplify this even further.  I’ll set this up for 20 MAC acres (which is about what I think the Kimley-Horn study used) and for 100 persons per acre under MAC (which is my best estimate for the average for the three approved housing-type MAC projects.)

In other words, this boils down to a simple question:  Suppose you took 2,000 people (20 acres x 100 per acre) and placed them in new housing on Maple.  How many do you think will drive to work (or to Metro) on a typical weekday morning?

When you put it that way, suddenly, the idea of considerably more traffic seems plausible, doesn’t it.  Particularly if you know that peak hour traffic is only 2366 vehicles per hour now.  It’s plausible because the short answer is “most of them”, and we can refine it from there.

So let me just fill in some vaguely plausible numbers for the calculation outlined above.   For the first two numbers, I’m sticking with 20 acres and 100 persons per acre.  For the last number — percent of rush-hour trips occurring in the peak hour — I’m taking a rough guess at 45%, based on eyeballing some national-average graphs.

For three rough cuts, here’s what the resulting increase in peak-hour traffic would be.  Upper bound — if you lose nothing at each step of the calculation — you end up with a 38% increase in peak hour traffic.  If you lose 10% at each step, then you get a 25% increase.  And if you lose 20% at each step, you end up with a 16% increase.

Obviously, the first one is an overestimate (or upper bound) by construction.  But, given that these are new buildings, geared toward younger people, I’d guess that the second one is fairly plausible.

In Vienna, only about 66% of adults work, but I believe that includes the retiree population.  Toss out retirees, and roughly 85% of non-retired Vienna adults work.  In Vienna, 79% of workers drive to work, but in addition, 10% take public transport, which, if you live on Maple, probably means driving to the Metro.  So roughly 90% of workers drive (or at least, start their commute with a drive).  And finally, if you start directly on Maple Avenue, there are few ways to get out of Vienna without driving on Maple, and even fewer ways that are faster than driving on Maple.  So I’m guessing that 90% of those living on Maple will have to drive, at least in part, on Maple, to make their AM commute.

Obviously, fill this in any way you want, and you can get any answer you want.  All I’m trying to say is that, given what I think I know about MAC buildings and Vienna commuting patterns, for the 20 acres or so modeled by Kimley-Horn, seems like a 25% increase in peak hour traffic is plausible, based on work commuting alone.  To that, you’d have to add driving to school, to day care, and to the new retail spaces (relative to what they replaced).

So, a one-third increase in peak hour traffic, for 20 acres redeveloped under MAC?  Could be I have messed up all three independent calculations I have done so far (this one, and the two I showed in Post #358).  But, barring that, something in that neighborhood seems plausible to me.