Here are some questions to ask yourself:

1.  How much growth (residential population and employment) has occurred and WHERE.  If there is a lot of growth, you should make new TAZs and not worry about keeping the same geography.

2. How big or small were the TAZs for CTPP 2000?  (In a few cases nationwide in 2000, the TAZs were very very small and the results were probably not reliable).  We have never controlled the size of TAZs, and even this time, it is a suggested threshold, not a firm threshold.  The software allows you to define very small TAZs, but gives you a warning.

You probably DON’T want to compare TAZ-to-TAZ flows between 2000 and the 2006-2010.  Most likely, I think that people will use the 2006-2010 CTPP TAZ-to-TAZ flows as a validation data source for their travel demand model. And in some cases, will use an adjusted TAZ-to-TAZ flow as inputs into HB-Work trips (after accounting for the fact that this represents only the journey-TO-work). 

For comparing between the 2 timeframes, one which is point-in-time and the other a “period estimate”, given the ACS sample size, you would be better off to make SuperTAZs (combinations of TAZs) to do the comparison. 

The UNWEIGHTED sample size after five years of ACS data collection (2006-2010) is about one-half the sample size from the Census 2000 long form, therefore the margin of error is much higher. 

We are asking MPOs (and State DOTs) to also define TADs, which is another word for a Super TAZs, using a minimum of 20,000 population.   This would be approximately the size of 4 or 5 census tracts.  For your comparison task, you might want SuperTAZs that are smaller than that. 

Glad to hear that you understand the difference between your model TAZs and the Census-TAZs defined for CTPP.

Elaine Murakami

FHWA Office of Planning

206-220-4460

From: ctpp-news-bounces@chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On Behalf Of Kendra Watkins
Sent: Wednesday, May 25, 2011 7:29 AM
To: ctpp-news@chrispy.net
Subject: [CTPP] taz delineation

Could someone shed some light on how reliable TAZ to TAZ flow comparisons will be between the 2000 CTPP data and the 2006 – 2010 release? I am asking because as I delineate TAZ’s I am trying to prioritize my “rules” for delineation and wonder if TAZ continuity will really be useful if the data won’t be comparable anyway.

Of course we are keeping our smaller travel model zones as consistent as possible, but our Census TAZ’s will be in many cases quite a bit larger.

Kendra Watkins
Senior Planner
Mid-Region Council of Governments
809 Copper Ave. NW
Albuquerque, NM 87102
Phone: (505)724-3601
Fax: (505)247-1753
Email: kwatkins@mrcog-nm.gov