Well, I’ve received a few responses
to my plea for direction, and I thank those who responded thus far.
Jonathan Lupton is correct, if anyone seems to have a reasonable handle on this
issue, please “chime in”. We don’t want to leave him
holding the bag with his “pedestrian knowledge” as the epitome of
the state of travel demand! ;-)
What I have developed to date is a TAZ
system that should reasonable provide us with enough detail to get a better
handle on forecasted volumes on the region’s “minor arterial”
network. In the past the focus had been primarily on the “principal
arterials”. In the next phase of this TAZ development, I plan to
aggregate this system into a nested system that could be used as a “fall-back”
in case the more detailed one resulted in large numbers of empty cells when
sent to the Census Bureau. This is where I find myself at the
moment. How many of the smaller TAZs do I need to combine?
Frank, your notes indicated 240 households
or about 600 people. I looked at that and found 39 percent of my TAZs fall
below that number when I “populate” the cells with year-2000 Census
data. Jonathan suggested 1000 population or employment. Again, looking
at my TAZs, I find 56 percent of them fall below THAT number. Fortunately
(I think), the problem TAZs are found overwhelmingly in the rural or developing
areas of our region OR they are TAZs in which I have large employment
concentrations. Given that scenario, I am attempting to aggregate these
TAZs into something larger. The question is, what should my minimum be?
Patricia, I agree with your thoughts on
the use of ACS data. However, in this instance, my concern is with the
upcoming 2010 census and the information we hope to obtain from a CTPP-like
product that has been urged by AASHTO and its Standing Committee on Planning.
Having dealt with CTPP (or UTPP) tabulations for several decades, this new
realm of ACS and LEHD apparently will be the way to go, but I am still hoping
that something will come out of the 2010 census that will reasonably
approximate that older product.
Thanks so far for the comments. I
hope they continue as we head into the decennial census year.
From:
ctpp-news-bounces@chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On Behalf Of Jonathan Lupton
Sent: Tuesday, August 04, 2009
4:32 PM
To:
Subject: RE: [CTPP] TAZ
Development
Mr. Paddock:
I bounced this same
question off Nanda Srinivasan years ago (ca. 2006) back when he was involved in
the CTPP. He seemed to have as firm a grip on this kind of issue as anybody. I
remember being told that the minimum TAZ size should be about 1,000 population
or 1,000 employment, to avoid disclosure problems. I do not know what would
happen in a TAZ that met one of these thresholds but not the other.
I am not offering
the information above as anything definitive; as best I remember there seemed
to be a lot of deliberate vagueness about appropriate TAZ size. I know our
TAZ’s in my region are presently quite a bit smaller than these
thresholds in most cases.
I would dare to
predict that LED may end up replacing CTPP in practical terms. At its “On
the Map” web site, LED already provides TAZ-level employment by detailed
industry for much of the country, including my region. LED doesn’t seem
to have a disclosure problem, although I think it doesn’t yet provide the
depth of journey to work data as in pt. 3 of the CTPP.
If anybody knows
more, please, please chime in. I’d hate to think that the pedestrian
knowledge of a peon like me represents the best information among the Great
Minds of transportation modeling.
Jonathan Lupton AICP
Research Planner
Metroplan
From:
ctpp-news-bounces@chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On Behalf Of
Sent: Tuesday, August 04, 2009
3:33 PM
To: '
Subject: [CTPP] TAZ Development
I must admit that, of late, I have gotten lost in the woods
while trying to see all the trees. During these past months I have been
engaged in developing a new TAZ system for the
This new TAZ system is substantially more refined than those
of the past and should result in reasonable travel loadings on our arterial
roadway network. However, certain issues have been raising their knobby
heads regarding zone size that probably will clash with Census Disclosure Board
rules. Does anyone have a reasonable idea as to what minimum population
or household levels a TAZ will need to attain in order to circumvent large
numbers of “blank” cells? I suspect that there is not an
easy, uncomplicated answer but would like to hear one nonetheless.
Transportation
Planning
Metropolitan
Council
Bob.paddock@metc.state.mn.us
651 /
602-1340