Subcommittee on Census Data for Transportation Planning
U.S. Department of Transportation 
Federal Highway Administration 
Federal Transit Administration 
Bureau of Transportation Statistics
C T P P  2 0 0 0  Special Report
June 1998
Tom.Mank@fhwa.dot.gov 
TAZ   Update   Program
Program Description 
Background  
Pre-Census Delivery of TAZs
Statistical Areas Program
Digital Submission Process 
Rules for TAZ 
Frequently Asked Questions
Questions and Contacts
 
 Program Description
In support of the Census Transportation Planning Package (CTPP) for Census 2000, the U.S. Department of Transportation (DOT) and the U.S. Bureau of the Census are asking for Traffic Analysis Zone (TAZ) delineations from Metropolitan Planning Organizations (MPOs) and individual state Departments of Transportation (DOTs).
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Background
Census 2000: Although the decennial census is still two years away, it is time to start planning for data products for the transportation community. The journey-to-work questions will be almost the same as in 1990, with small exceptions in coding categories for number of persons in carpools, and number of vehicles in the household. The journey-to-work questions are part of the census "long form." The Bureau of the Census expects that Census 2000 will be the last time the "long form" is used, with the American Community Survey to replace it before 2010.

CTPP 2000: In 1990, the CTPP data product was financed through a "pooled fund" project administered through AASHTO for a "penny per person." We hope that a similar funding mechanism will allow the development of both statewide and urban elements for CTPP 2000. DOT is currently eliciting input on standard tables and approaches to customize tabulations for CTPP 2000.
 

Date
Task
Early 1999
CB sends out TIGER/Line 1998® 
Early 1999
DOT sends out TAZ software 
Summer 1999
MPOs and State DOTs submit TAZ boundaries to CB Regional Offices 
June -Dec. 1999
CB enters TAZs into TIGER ® 
Jan. 2000
CB returns TIGER/Line 1999® to MPOs and State DOTs for 1 month review 
Table 1: TAZ Update Program Schedule
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Pre-Census Delivery of TAZs
In the past, TAZs were defined to include a single census block/tract, multiple census blocks/tracts, a place, a county subdivision, or an entire county. For 1990 delineations, TAZs were required to follow these census designated boundaries. This meant that MPOs / DOTs could not define TAZs until after the decennial census boundaries were released.
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Statistical Areas Program
NOTE: MPOs/DOTs are strongly encouraged to work cooperatively with their local Statistical Areas Program participant regarding potential changes to their area's block group, census tract, census designated place and census county boundaries. For areas that desire to maintain coextensivity between TAZs and standard census geography, this program is the place to have your concerns voiced. Areas that don't plan to submit TAZs will also likely benefit from coordinating with those involved in this program.

The Census Bureau isn't planning to number the census blocks for Census 2000 until the Fall of 2000. So it is important for MPOs/DOTs to note that census blocks and other Census 2000 tabulation boundaries will not appear on the issued TAZ delineation maps discussed above. The 1990 boundaries may be included on these maps, if requested, as reference geographic boundaries.

Participants in the Census Bureau's Statistical Areas Program began receiving delineation maps and guidelines in early 1998. Census Bureau Regional Offices can provide the MPO/DOT with the name of their jurisdiction's contact. A list of Regional Offices is included in the June 1998 CTPP 2000 status report.

For the Census 2000 TAZ Update Program, MPOs and DOTs are being asked to define their TAZs prior to the census - enabling MPOs/ DOTs greater flexibility in defining their TAZs and an earlier release of the CTPP.

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Digital Submission Process
The U.S. DOT is developing a process for the submission of TAZ boundaries to the Census Bureau using digital files in combination with TIGER/Line 1998®. We believe that a digital process will be more accurate and less costly than a paper process, therefore paper submissions will be available only on a limited basis.

The Census Bureau will provide TAZ Delineation Maps and guidelines to MPOs/DOTs in early 1999. MPOs/DOTs will have the choice of receiving the delineation maps in digital format or as paper maps. Participants will have six months in which to delineate their TAZ boundaries and return the materials to the Census Bureau regional office. As the data are received, they will be added to the Census Bureau's geographic data base.

Beginning in early 2000, the Census Bureau will provide verification maps or digital files to each MPO/DOT for them to verify that their respective TAZs were inserted correctly before being used in Census 2000.

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Rules for TAZ
As with previous programs, each MPO / DOT is required to provide a "complete" TAZ coverage for a county. That is, all areas of a county must have an assigned TAZ. This may include both "true" TAZs (those areas actually under study by the MPO / DOT) and "dummy" TAZs (those portions not included within an MPO / DOT's area of study). Each TAZ will be identified by a unique six character alphanumeric code within the county.

To be part of the TIGER file, each county can only have one definition of TAZs, so if an area falls within the planning boundaries of multiple MPOs, the MPOs must work together to define a single set of TAZs for TIGER.

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Frequently Asked Questions

Do I have to define Traffic Analysis Zones for the CTPP?
No, you can get the Census Transportation Planning Package (CTPP) without defining TAZs. You can use standard census geography, like census tracts or block groups. You will need to make a decision by early 1999, when the TAZ boundary definition for CTPP 2000 begins.

What are the advantages to defining TAZs?
TAZs are designed specifically for use with travel demand models. Therefore, factors such as business concentrations and traffic flow are considered in their design. Since census tracts and block groups are defined to count residential population, they may not be the best geographic basis for transportation analysis in commercial portions of a metropolitan area. For example tract # 1115 in Figure 1 is a single block group but contains 10 TAZs. While tract # 1113.08 is a single TAZ but is composed of 3 block groups.

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What are the advantages of using standard Census geography rather than TAZs?
You will be able to have data tables from standard Census products for the same geography. In previous decades, the Census Bureau issued STF tapes and CDS. For Census 2000, it is likely that tables similar to STF will be accessible through the Internet using a project currently called Data Access and Dissemination System (DADS) for census tracts and block groups. Then, when the CTPP tables are issued, these tables can be directly matched.

How long (how many hours) will it take my MPO to enter the TAZ definitions to give to the Census Bureau?
It depends on:
    1. How many TAZs you have.
    2. The type of process you choose, a paper map or GIS software based process.
    3. The present status of your TAZ boundary file. Do you already have TAZs defined that you will want used in CTPP 2000, or will you have to go through a review process to adjust TAZ boundaries for CTPP 2000?


Questions or Comments
Todd Blair
Geography Divison
Bureau of Census
(301)-457-1099
Tblair@quickmail.geo.census.gov
Tom Mank
FHWA, DOT. (202)-366-4087
Tom.Mank@fhwa.dot.gov
We estimate that an area using the GIS based software you could update or create about 15 zones per hour. 
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