Ed is correct, a concept of "Super TAZs" may very well happen with the
2010 Census, in addition of course to "Base TAZs". A Super TAZ would be
an aggregate of Base TAZs (or census tracts or block groups), with a
threshold of probably 20,000 population. "Base TAZs" may feature a
threshold of 600 persons (residents or workers), but like Ed said, if
NCHRP 8-79 proves to be successful, then there may be no reasons for a
minimum TAZ threshold per say. The idea behind all this is that one
could perform socio-economic analysis and forecasts at the "Super TAZ"
level (with proper disaggregation to the "Base TAZ" level), while
detailed traffic assignment could take place directly at the "Base TAZ"
level. More on TAZ disaggregation methods and techniques can be found
The shape and size of zones, the way the zones are delineated, and how
well the zones integrate with the highway network all influence the
representation of highway and transit access in a model. One way to
assess the reasonableness of zone systems (with respect to highway and
transit access) is to determine the "optimal" zone size for which the
highway and transit access procedures are created.
Also important is how the zone system size corresponds to activity
patterns. More concentrated activity necessitates smaller zones in those
areas, as the increased activity requires the reduced geographic unit to
minimize the amount of intra-zonal movements and maintain consistency
among trip distribution characteristics throughout a model area.
Activity levels vary over time, so both the base and future year
activity levels should be compared with their zone sizes.
I hope this helps for now, more guidance to follow in the fairly near
Traffic Modeling Manager
Atlanta Regional Commission
[mailto:firstname.lastname@example.org] On Behalf Of Ed Christopher
Sent: Thursday, August 06, 2009 10:45 AM
Subject: Re: [CTPP] TAZ Development
Watching this discussion on TAZs has been interesting and the size of
TAZs has been shown to be more of an art than a rule based science.
With that said I want to toss a few more items on the table when folks
are considering TAZs.
1. TAZs to be used in the production of a CTPP 5-year ACS product will
not have to be developed until the Summer of 2011. The process used
will be an equivalence method where folks will be asked to say which
Census Blocks (in their entirety) will be placed in which TAZ.
2. There will likely be at least two TAZ sizes. The regular TAZs that
we have all come to know and the subject of Bob's question and maybe a
larger one somewhere in the neighborhood of 20,000 residents. Just this
week the AASHTO CTPP Oversight Board established a subcommittee to work
out the TAZ details. Guy Rousseau of the Atlanta MPO is chairing the
3. The TAZs will be developed to support a tabulation of the 5-year ACS
data using (2006, 07, 08, 09 and 2010) data that is planned to be
released in the Summer of 2012. For areas that do not submit TAZs
Tracks (and maybe Block Groups) will be the default.
4. For the CTPP 5-year tabulation there is a NCHRP project (NCHRP 8-79)
getting underway that, if successful, will take any disclosure issues
off the table. The RFP for the project, "Producing Transportation Data
Products from the American Community Survey that Comply with Disclosure
Rules" was just finalized and put out on the street two days ago. See
to view the
RFP and details.
5. Assuming NCHRP 8-79 is successful, which there is no reason why it
should not be, TAZ sizes will not be affected by the artificial
disclosure rules and instead can go back to being determined based on
good planning practice. Which doesn't answer Bob's original question
but does help put the focus where it should be.
6. Regarding disclosure rules we know from our attempt to get some CTPP
tables produced from the 3-year ACS that if the same rules for the
3-year were extended to the 5-year data the losses would be so
devastating that there would be no usable 5-year data, which is why we
fought for the NCHRP project.
7. In terms of a 3-year CTPP ACS product we hope to have one released in
the Spring/Summer of 2010 using 2006, 07 and 08 data. This release will
be subject to a variety of data suppression and disclosure rules and
will only be for areas of 20,000 residents or more. To see the tables
that were designed for this go to http://trbcensus.com/products/
8. In terms of TAZ sizes for planning the Florida DOT through its
modeling group has published a report on the subject. I do not have the
link to the report handy but maybe someone else the list can find it and
I hope this helps clarify a few things
Paddock, Bob wrote:
I must admit that, of late, I have gotten lost in the
to see all the trees. During these past months I have
been engaged in
developing a new TAZ system for the Minneapolis - St.
Paul MPO area
will both serve the needs of travel demand modeling
here at the
Metropolitan Council *AND* provide us with the information we want
a 2010 CTPP-like product.
This new TAZ system is substantially more refined than those of the
and should result in reasonable travel loadings on our
network. However, certain issues have been raising
their knobby heads
regarding zone size that probably will clash with
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
an easy, uncomplicated answer but would like to hear
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