I will be on vacation from August 10 through Sept 3, 2009.
If you need immediate help please contact:
Tanya Rodriguez at 212-383-2516 (trodriguez(a)dot.state.ny.us)
Munnesh Patel at 212-383-2528 (mpatel(a)dot.state.ny.us)
Jorge Argote, P.E.
Manager, Travel Surveys Unit
Technical Group
NY Metropolitan Transportation Council
199 Water Street, New York, NY 10038
Tel: (212) 383-2527; Fax: (212) 383-2418
jargote(a)dot.state.ny.us
The seminar I went to two years ago seemed to indicate minimum TAZ size in the range of 1200 people (again, pop or employment or combination). The issue is that in most areas transit accounts for 0.5-1.5% of the mode split and if you want to keep from having that data suppressed you will need to have that many people in it. I was told that suppression usually will occur when the real value is 3 or less, but true integer value reporting doesn't start until the real value is more than 10 (I think...it's been 2 years). The problem then becomes that area totals over several TAZ's are way off because you are adding "rounded" numbers rather than real numbers.
I suspect that the real question will have to do with which data set you are going to use. If you are going to try to use the ACS 1 year aggregate you really need large TAZ's. They say they are not going to suppress as much in the 3 year aggregate and may not suppress anything in the 5 year (at least for category totals).
As a modeler, this drives me completely crazy. To get the network to load well, you need to have the TAZ size and connections that are appropriate relative to the geography and roadway network. If you're dealing with a subdivision with 500 houses and one or two driveways the 1200 people limit is fine. If you're dealing with a city block with single family units, you've got a real problem.
One way to get around this (thinking out loud) is to take an area with a relatively uniform density and report that area as a whole as a census TAZ. Then when you get the data back, you can split that TAZ proportionately for your model's sake. Most travel demand models are only accurate enough to get you lane calls anyway so making some internal estimates from the Census data will keep you away from suppression issues and not adversely impact your accuracy. It will take a little data massaging on the back end but it may be worth it, especially in areas with dense roadway network connections.
Patricia C. Tice, PE, AICP, LEED AP
Glatting Jackson Kercher Anglin, Inc.
120 N. Orange Ave
Orlando, FL 34787
407-284-4753
I recently had a chance to talk with a Census Bureau staff and was told
that the 3 (not 5) year average of CTPP like product will be available
in fall 2010.
Kyeongsu Kim
Census & Travel Survey Specialist
NY Metropolitan Transportation Council
199 Water St, 22nd fl.
New York, NY 10038
kkim(a)dot.state.ny.us
1-212-383-7233
>>> ptice(a)Glatting.com 8/5/2009 11:56 AM >>>
If the information that I received two years ago is correct, the ACS
will be the only thing available. Unless something has changed, ACS
replaces "long form" and it will not be used in the 2010 census.
However, 2010 will see the first publishing of the 5 year running
summary from the ACS. I assume a CTPP like product will be (or at least
could be) generated from this 5 year summary.
Again, if someone more who is more up to date, I would love to know
where this stands.
Patricia C. Tice, PE, AICP, LEED AP
Glatting Jackson Kercher Anglin, Inc.
120 N. Orange Ave
Orlando, FL 34787
407-284-4753
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If the information that I received two years ago is correct, the ACS will be the only thing available. Unless something has changed, ACS replaces "long form" and it will not be used in the 2010 census. However, 2010 will see the first publishing of the 5 year running summary from the ACS. I assume a CTPP like product will be (or at least could be) generated from this 5 year summary.
Again, if someone more who is more up to date, I would love to know where this stands.
Patricia C. Tice, PE, AICP, LEED AP
Glatting Jackson Kercher Anglin, Inc.
120 N. Orange Ave
Orlando, FL 34787
407-284-4753
I will be out of the office starting Wed 07/29/2009 and will not return
until Mon 10/05/2009.
I will respond to your message when I return.
Please contact with Pramoda Gode (pgode(a)camsys.com) for any CTPP related
questions.
Mr. Paddock:
It's safe to say that one set of TAZs will not be able to cover both these
needs for you. There's been discussion here before about developing two
or more sets of TAZs just for different levels of disclosure problems with
2010 CTPP (ACS) data alone. You'll need at least one more set for traffic
modeling, where you can geocode the address-based, Census block and other
small area data not subject to these disclosure problems and than from
there aggregate to the level needed for the model.
Sam Granato
Ohio DOT, Office of Multi-Modal Planning
1980 W. Broad Street, Columbus, OH 43223
Phone: 614-644-6796, Fax: 614-466-0822
"It's all very well in practice, but it will never work in theory." -
French proverb
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
