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CTPP Status Report

April 2004

Update on Data Release

On Feb. 19, 2004 the Census Bureau (CB) finished releasing preliminary Part 2 data in .ivt format to state DOTs and MPOs. The preliminary Part 2 data are also available in flat ASCII files on the BTS website. To download, go to, and click on CTPP 2000.

Final Part 1 data in flat ASCII files are available for download at the same BTS site. CB expects to start shipping final Part 1 CDs in .ivt format with software to state DOTs and MPOs around the end of April. Shortly thereafter, BTS will process orders for CDs from the general public at htpp:// (click on Products).

Preliminary Part 3 data in flat ASCII files are expected during April 2004 as well. Download from the BTS Transtats site will be available. CDs with the .ivt version and software are not expected until May.

For questions on the flat ASCII files, contact Clara Reschovsky at 301-763-2454 (e-mail

For questions on the CTPP Access Tool or to comment on the data, please contact Nanda Srinivasan at 202-366-5021 (e-mail:

Journey to Work: Census 2000 Brief

by Elaine Murakami, Federal Highway Administration

The Census Bureau has finally (!) released the Journey to Work: 2000 Brief. This was written by Clara Reschovsky, who has been a key Census Bureau staff in the production and distribution of the CTPP 2000. Many of you have talked to Clara by phone or email for technical questions related to TAZ definition, MPO region definition, determining the number of copies of CDs to be mailed, ad infinitum. We are proud that in addition to her work at the Census Bureau, Clara has been working on her Master's degree, and expects to finish in May 2004 with a degree in Transportation Policy from George Mason University.

Some of the results are:

The proportion of the U.S. population living in metropolitan areas increased from 1990 to 2000 by 13.1 million (from 93.1 million to 106.3 million). However, for the first time, more than half of metropolitan area resident workers worked outside of the central city portions of the metropolitan area.

Commuting to Downtown Study

By Ed Christopher (Chair, TRB Committee on Urban Transportation Data and Information Systems) (ABJ30)

The Transportation Research Board (TRB) Committee on Urban Transportation Data and Information Systems (ABJ30 formerly A1D08) wants your help to build "downtown profiles" for the largest cities in the United States.

The "downtown profile" is envisioned as a two-page product: one page with a half-page downtown locator map and a half-page table showing commuters by means of transportation to work. The second profile page would include a narrative summary.

Table 1 shows some of the data to be included. Note that the worker count is from Census workplace results, and is not adjusted for total employment. The value of the profile is that it includes a time dimension in addition to the distribution of travel modes, and total workers and employment.

Figure 1 illustrates the share of transit for downtown commutes. Although transit commute mode share for the entire U.S. is around 5 percent, transit plays a critical role in commuting to downtowns. In several cases, transit shares have increased since 1990, returning to rates similar to those found in 1980.

Chuck Purvis (Metropolitan Transportation Commission, Oakland) is coordinating the effort. For an abstract of the study, please visit and

Please participate! For more information, please contact Ed Christopher ( or Chuck Purvis (

Figure 1. Transit Shares to Downtown in 2000

Bar chart: Percent of workers at work. Chicago, 62%. San Francisco, 49%. Seattle, 37%. Oakland, 24%. Baltimore, 19%.

Table 1. Mode Shares to Downtown for Selected Cities

City Year Census Workers Ride share Transit
Chicago, IL 1980 353,984 8.6 72.9
1990 321,978 9.5 61.0
2000 341,014 8.0 61.7
Seattle, WA 1980 72,703 16.6 43.7
1990 94,421 13.7 34.3
2000 118,090 13.3 36.9
San Francisco, CA 1980 279,300 16.0 51.4
1990 284,300 14.9 46.6
2000 320,300 12.4 49.0
Oakland, CA 1980 53,500 17.6 26.0
1990 51,900 13.3 20.2
2000 63,200 12.8 24.1
Baltimore, MD 1980 121,990 22.1 31.7
1990 127,680 17.1 24.1
2000 116,340 13.3 18.6

Data Rounding in CTPP 2000

By Nanda Srinivasan, Cambridge Systematics Inc.

All CTPP tables (except mean, median, and standard deviation tables) are subject to the following rounding rules:

Columns for Totals in each table may not match the sum of the categories because Totals are rounded independently of the cells. For example:

Total households= 39 is rounded to 40 (and NOT 4+15+10+10+4 = 43 rounded to 45).

However, because some variables (Example: Mode to work) are classified in more than one way, the number of possible answers is higher. Chuck Purvis (see Table 1) (Chuck Purvis, Metropolitan Transportation Commission, Oakland, California, e-mail posted to the CTPP-news listserve on February 19, 2004) notes that sometimes, up to 15 answers may be possible!!

Table 1. Different Tables and Different Geographies yield (slightly) Different Answers!

Total Table 2-2 Table 2-12 Table 2-27
Sumlev Transit=5 categories Transit=3 categories Transit=2 categories
4031 TAZ 319,435 319,553 319,600
4384 Blk Grp 319,433 319,521 319,541
1403 Tract 319,717 319,780 319,836
9 County 320,116 320,129 320,125
1 MPO 320,125 320,120 320,120

Because each table may provide a slightly different answer, we recommend that where possible; try-obtaining totals from published CTPP tables, rather than aggregating geographies, or individual table categories.

Rounded Values vs. Unrounded Values

A comparison of CTPP 2000 data and Summary File 3 data showed that the CTPP estimate was more likely to be LOWER than the SF 3 value. We noticed that the rounding value of 1-7 to 4 generally provides a lower estimate than the actual value. This occurs because:

Which value to use: Totals or "Sum of the Parts"?

Rounding does not affect the "statistical" significance of the data. For example, there is no difference between 431,899 and 431,753. The difference of 136 is well within the sampling error. The choice of using published totals versus summing the different parts of the table is left to you. If you are reporting percentage distributions, we would suggest you use the published totals as the denominator. If you are making a pie or a bar chart, you could use the "Sum of the parts" as the total.

Aggregating Geography: How to Make Allowances to Rounding?

When you want to aggregate geography, first check to see if there is a summary level in CTPP 2000 that contains the aggregated geography. For example, there is no need to aggregate data from tracts or TAZs to county geography because county data are published as a summary level in CTPP. Table 2 compares the sum total of tracts, and TAZs with the published totals for Mode to Work (by Place of Work) for Montgomery County, MD. Because the county is divided in to 117 tracts, and 1124 TAZs, the TAZ totals are off more than the tract totals. However, the percent of transit commuters is the same in all three cases.

Table 2. Mode to Work by Place of Work, Montgomery County, MD

Published Values Sum of Tracts Sum of TAZs
All workers 420,875 420,865 420,757
Drove alone 308,215 308,230 308,130
2-person carpool 37,280 37,307 37,180
3-person carpool 6,745 6,720 6,666
4-person carpool 2,300 2,294 2,282
5-6 person carpool 1,020 994 1,002
7-or-more carpool 825 802 804
Bus or trolley bus 19,000 19,016 18,959
Streetcar or trolley car 150 150 141
Subway or elevated 9,900 9,887 9,839
Railroad 1,045 1,032 1,043
Ferryboat 10 10 10
Bicycle 935 929 900
Walked 8,880 8,847 8,814
Taxicab 500 490 486
Motorcycle 235 220 219
Other means 1,900 1,853 1,854
Worked at home 21,935 21,915 21,802
Percent Transit 7.15 7.15 7.13

Aggregating to an Unpublished Geography

To aggregate data to an unpublished geography (example: Central Business District - CBD), consider the LARGEST geography that can be used to aggregate. For example, if the CBD can be defined using Tract geography, then use Tracts instead of using more finely defined TAZs. This will minimize distortions introduced by rounding.

Figure 1 shows how 2000 TAZ, and 2000 Tract boundaries can be assembled to obtain the 1990 CBD definition for Seattle, WA. 21 TAZs OR 4 Tracts approximately equal the 1990 CBD geography. Table 3 shows that both the TAZ and Tract results are close, even with rounding and geographic differences.

How Bad is Rounding?

Chuck Purvis notes in his post to the CTPP list serve, "The rounding of values inside the CTPP is, right now, a modest, annoying data processing issue. As professional data analysts, we are always on the lookout to make sure our numbers "add up" so that we're not missing anything." Chuck would prefer that the CB use a different approach to disclosure avoidance that results in numbers that add up. Chuck's final recommendation (Chuck Purvis, Metropolitan Transportation Commission, Oakland, California, e-mail posted to the CTPP-news listserve on February 19, 2004) is to "Develop a sense of humor... Don't take these data issues too seriously. And it's kind of funny that the numbers don't add up. Or, as they say: "close enough for government work."

Figure 1. Combined TAZs/Tracts Overlaid on 1990 CBD Definition - Seattle, WA

The left side map shows the equivalence of Tracts to CBD for Seattle. The right side map shows the equivalence of TAZs to CBD for Seattle.

Table 3. Comparison of Tract, and TAZ based definitions of CBD

Geography Total Workers Drove alone Carpooled Transit
Seattle City Total 477,240 278,645 61,840 82,305
CBD: TAZ based definition 118,090 48,315 15,675 43,532
CBD: Tract based definition 104,360 41,885 13,645 38,919
TAZ based definition: CBD share of Seattle City 24.7 17.3 25.3 52.9
Tract based definition: CBD share of Seattle City 21.9 15.0 22.1 47.3

Race/Hispanic Origin Tables in the CTPP 2000

By Elaine Murakami, Federal Highway Administration

CTPP 2000 has a wealth of tables that include Race and Hispanic origin. CTPP from previous decades virtually ignored these characteristics. However, at the time of CTPP table design, environmental justice issues were prominent, resulting in the addition of tables using these variables, and also poverty status (Table 1).

Table 1. Race by Hispanic Origin Tables in CTPP Parts 1 and 2

Table name
1-23 Hispanic origin by Race by Occupation Workers
1-24 Hispanic origin by Race by Industry Workers
1-25 Hispanic origin by Race by Class of Worker Workers
1-26 Hispanic origin by Race by Worker Earnings Workers
1-27 Hispanic origin by Race by Means of Transportation Workers
1-28 Hispanic origin by Race by Travel Time to Work Workers
1-44 Household income by Hispanic origin by Race by Means of Transportation Workers in household
1-45 Vehicles available by Hispanic origin by Race by Means of Transportation
1-46 Poverty status by Hispanic origin by Race by Means of Transportation
1-80 Vehicles available by Race by Hispanic origin Households
1-81 Race by Hispanic Origin by Telephone Avail Households
1-82 Household size by Household Income by Race by Hispanic Origin Households
2-23 Hispanic origin by Race by Occupation Workers
2-24 Hispanic origin by Race by Industry Workers
2-25 Hispanic origin by Race by Class of Worker Workers
2-26 Hispanic origin by Race by Worker Earnings Workers
2-27 Hispanic origin by Race by Means of Transportation Workers
2-28 Hispanic origin by Race by Travel Time to Work Workers
2-44 Household income by Hispanic origin by Race by Means of Transportation Workers in Households
2-45 Vehicles available by Hispanic origin by Race by Means of Transportation
2-46 Poverty status by Hispanic origin by Race by Means of Transportation

Table 2 (obtained from CTPP 2000 for San Diego County, CA) clearly shows how different Hispanic households are, with over 50 percent of households with 4 or more persons, and over 40% with income below $25,000. White, non-Hispanic households on the other hand were dominated by 1 and 2-person households, with 43 percent with incomes above $60,000.

Table 2. Household Size by Household Income by Race by Hispanic Origin: San Diego County
Household Income in 1999 All Households 1-person household 2-person household 3-person household 4-or-more-person household
White alone, Non-Hispanic All Households 657,280 187,575 243,260 98,530 127,920
Income < $25k 173,180 96,595 48,215 14,715 13,660
Income between $25-60k 199,505 58,380 80,590 28,770 31,770
Income equal to or more than 60k 284,590 32,600 114,455 55,040 82,500
Black Alone, Non-Hispanic All Households 53,365 13,320 13,630 10,165 16,250
Income < $25k 21,650 8,170 5,415 3,610 4,460
Income between $25-60k 18,175 4,085 4,800 3,360 5,925
Income equal to or more than 60k 13,540 1,060 3,420 3,195 5,865
All Hispanic All Households 182,120 21,585 33,130 31,695 95,710
Income < $25k 77,760 14,825 13,820 14,175 34,940
Income between $25-60k 61,495 5,110 11,455 10,075 34,855
Income equal to or more than 60k 42,865 1,650 7,850 7,445 25,920

Source: CTPP 2000, Table 1-82

Similarly, Summary Files 3 and 4, standard products from the Census Bureau have a wealth of tables in race and Hispanic detail. The SF 3 tables are limited to census tract reporting, called PCT and HCT. Among the PCT and HCT tables in SF3 are two of particular interest to transportation planners, PCT65 (Means of Transportation to Work) and HCT33 (Vehicles available). The tables are structured as follows: PCT##

  1. White alone, (Hispanic and non Hispanic)
  2. Black alone
  3. American Indian and Alaska Native alone
  4. Asian alone
  5. Native Hawaiian and Other Pacific Islander alone
  6. Some other race alone
  7. Two or more races
  8. Hispanic, all races
  9. White Alone, non-Hispanic

A large proportion of the Hispanic population selected "other" as their race.

Mode to work in 2000 has important differences by race and Hispanic origin. Many of these differences are correlated with differences in household income, household size, and residential location. Data from Summary File 3 Table PCT65 shows (See Table 3):

Table 3. United States: Mode by Race by Hispanic Origin

Mode White, non-Hisp Black Hispanic
Drive Alone 80% 66% 62%
Carpool 10% 16% 22%
Transit 3% 12% 8%
Walk 3% 3% 4%
Work at Home 4% 1% 2%
Other 1% 2% 2%
Total 100% 100% 100%

Source: Census 2000 Summary File 3, Tables PCT 65 b, h, and i.

Census Data Subcommittee at TRB Annual Meeting

By Ed Limoges, Sabre Systems, Inc.

The Subcommittee on Census Data,

ABJ30 (1),met on January 14, 2004 at the 2004 Annual Meeting of the Transportation Research Board. Over thirty persons attended, representing Federal and State agencies, MPOs, consulting firms, and academia. The meeting was chaired by Bob Sicko. Here are a few highlights from the meeting. The complete notes are posted at

American Community Survey

Jay Waite, Associate Director for Decennial Census and American Community Survey, presented an update on plans and activities for Census 2000, including ACS.

The nationwide ACS will start in July 2004. The earliest that small geographic reporting, e.g. block group, would be available would be an accumulation of 2005 through 2009 data, with data release toward the end of 2010. The five-year accumulated data will be published in the geography of the last year, so that 2010 data would reflect annexations up to that year. Census staff is currently working on workplace coding options.

There will be a process for modifying the questionnaire content with any changes taking place with the 2008 collection year. Census Bureau will meet and discuss ACS issues with staff from each of 21 Federal agencies.

NCHRP 08-48

Kevin Tierney, Cambridge Systematics, discussed the ACS NCHRP project. The purpose of the project is to develop a guidebook for transportation planners about how to use ACS, adjust for the loss of the long form, and to identify new uses for the data. A draft of the guidebook is expected to be available for the Spring 2005 TRB Census data conference in Irvine, CA.

Elaine Murakami stated that to augment the NCHRP project DOT contracted with the CB for a special tabulation of the 3-year test ACS data. The tabulation is designed as a "mini CTPP" with a Part 1, Part 2 and Part 3 set. Although the data tabulation was developed to support the research associated with the NCHRP project, the data will also be made available to interested researchers.

CTPP 2000

Phil Salopek, Census Bureau, presented an update on the production and distribution for CTPP 2000.

Public Use Microdata Sample (PUMS)

Larry Blain, PSRC, described his use of PUMS data for cross tabulation, and urban simulation. Ken Cervenka, NCTCOG, will do reasonableness checks on the worker flow data, and use PUMS to analyze vehicle availability. Chuck Purvis, MTC, distributed a one-page handout describing travel demand modeling applications and exploratory research.

Updated: 10/20/2015
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