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 group.
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 http://www.trb.org/TRBNet/ProjectDisplay.asp?ProjectID=2708 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 post it.
I hope this helps clarify a few things
Paddock, Bob wrote:
------------------------------------------------------------------------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 Minneapolis – St. Paul MPO area that will both serve the needs of travel demand modeling here at the Metropolitan Council *AND* provide us with the information we want from a 2010 CTPP-like product.
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.
*Bob Paddock***
Transportation Planning
Metropolitan Council
Bob.paddock@metc.state.mn.us
651 / 602-1340
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Ed Christopher
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FHWA Resource Center Planning Team
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