Traffic and the ultimate impact of MAC zoning

Posted on July 26, 2018

Citizens have repeated pointed to traffic as a concern, and to the fact that the Town apparently has done no long-range analysis of what MAC is likely to do to traffic.  On this page, I’m going to take the traffic analysis for Tequila Grande/444 Maple West, scale it up to the (roughly) 70 acres that the Town projects will eventually redevelop under MAC, and estimate the number of new vehicle trips on Maple, once all that redevelopment has taken place.

Scroll down to the tables below if you just want the results.

The analysis of traffic really points up a major shortcoming of the Town’s zoning-enforcement approach to MAC.  The Town is treating each MAC project as a one-off zoning request.  So they get a stand-alone analysis of traffic, separately for each property.  And — unsurprisingly — each one has (and likely will) come back saying that they add just a little bit to the existing traffic.  And that’s completely true — each individual project adds just a bit to the stream of traffic on Maple.

To an economist, traffic in general, and this instance in particular, is a classic “tragedy of the commons“.  Each building contributes its nearly-harmless little bit to traffic.  But in total, we’re going to congest Maple even further.  In a situation like that, it’s the government’s job to assess the overall impact of all the development taken together.  That’s the gap I am trying to fill here.

It’s easy enough to get a crude estimate of additional car trips on Maple.  First, we know the current traffic counts, courtesy of the Virginia Department of Transportation.  See here if you want to read the background on where these numbers come from.  To that, I’ll take the traffic analysis of 444 Maple West, and simply scale it up to the roughly 70 acres that the Town thinks are amenable to MAC development.

Our starting point:

On a given work day, the center of town sees about 33,000 vehicle trips, counting all trips in both directions of travel.  On a weekend, the count drops to about 22,500.  But as any Vienna resident will tell you, the weekend traffic makes up in chaos what it lacks in sheer volume.  While the weekday commuter traffic is “linear” in the sense that most people want to go in the same direction, on the weekend, everybody travels in every direction.  And the result tends to be some long travel times on Maple.

The impact of 444 Maple West:  The February 16, 2018 traffic analysis supplied by the developer lists an average 1,446 average weekday trips.  Some of those would be via Nutley, but that’s unique to this property.  For purposes of an estimate for all of MAC, I’ll assume all trips will be on Maple.  They do not list average weekend day trips.  I’m going to assume that would be in the same ratio as all of Maple, yielding 986 weekend trips.

I’m also not offering any judgment as to accuracy of these estimates.  These are based on some average US data, and they have discounted the raw trip estimates due to various aspects of the proposed building (e.g., they have bicycle parking.)

Extrapolate to all acreage likely to be developed.  Prior statements by a Town Council member said that the Town expects 70% of the 106 MAC acres to be developed.  The 444 Maple West development is just about 2.76 acres.  So if everything develops at the 444 Maple West density, total trips would be (roughly 70/2.75 =) about 25 times what we’ll expect from 444 Maple West.

First, an overestimate:  The results from that initial calculation are shown in the first set of numbers below.  If this one project — 444 Maple West — generates 1,446 weekday trips, and the entire area likely to be developed is 25 times larger, then the simple arithmetic says that the entire area likely to be developed under MAC would more produce more trips that currently occur on Maple.

But that’s not right, as a way to estimate the net impact — the new trips.  The main issue is that I have not netted out traffic to the existing retail sites.  Every MAC building will replace current commercial space with new commercial space.   In theory, those trips to the new retail sites will just replace trips that would have been made to the old retail sites.   (In practice, see below, there is likely to be some expansion of those trips.)

Second, a residential-only estimate, ignoring the commercial space:   I should develop a much more conservative estimate based only on the trips from the new housing on Maple.  Since there is no housing on Maple now, we can pretty clearly call all of those additional trips that would not be seen now.   There, the developer’s estimate, discounted for various features of the property, is 960 trips on the typical weekday.  The second set of numbers below redoes the estimate starting from there.

There needs to be a further upward adjustment for the conversion of current office space to retail under MAC.   Proposed changes to MAC would require that any current first-floor commercial space be retained as commercial space.  Because the vacancy rate for office space in Northern Virginia is quite high, that mandate will likely result in some expansion of retail space (because, basically, what else can they build?)

For now, ignore the details and focus on the big picture:  MAC development at the density proposed for 444 Maple West will substantially increase traffic on Maple in the long run.  If every trip for residents begins, ends, or passed through Maple, you’d see a roughly 78% increase in traffic.  For me, that’s almost impossible to grasp.

Sensitivity analysis:  Recall the assumptions behind this:  This accepts the developers (discounted) estimate for residential traffic, it assumes all trips from these residence involve Maple Avenue in some way, it assumes that the Town’s effort to boost retail in Vienna is a total flop (no change in trips due to the new retail space), and it assumes that 70% of the total MAC-zoned property would be developed at the 444 Maple West density.  Let me informally change a few of those things, and just line out the resulting change in traffic, without showing the calculation.  (Apologies to those seeing this on a a phone – may not be legible).

The bottom line here is that under any reasonable set of assumptions, this is a significant and material issue, and the Town need 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.

There are some caveats here, but they mostly seem to point in the direction of understating the amount of traffic at that site.  For example, traffic analyses routinely exclude an estimate of so-called “bypass” trips.  These are people who actually drove into and out of the site, but were presumed to be already on Maple Avenue doing something else.  Traffic engineers don’t count that fraction of trips as additional traffic.  So the projected number of vehicles going in and out of the entrance to the new building will substantially exceed the estimate of additional traffic presented above.  If you live next to the new building, those additional trips into and out of the parking lot might matter to you.

Still to be done:  Peak hour traffic.   A colleague familiar with the traffic data pointed out that the impact on “peak hour” traffic appears to be much larger than the impact on daily average traffic.  It’s during that “peak hour” that you really experience traffic jams on Maple, so that’s probably a more important thing to look at.

Traffic engineers caution against looking too closely at that issue because the “peak hour” for a shop or building may or may not coincide with the peak hour for Maple Avenue as a whole.   (E.g., traffic to a breakfast-oriented restaurant and a dinner-oriented restaurant will have different peak hours.)  That said, the point is well taken, and I plan to extent this analysis to look, at least nominally, at the issue of peak hour traffic.