Emerging Issues for Transportation Data Users of

ACS

Elaine Murakami, FHWAElaine.murakami@fhwa.dot.gov

TRB Planning Applications Conference, April 26, 2005

First, the good news…

• CB has added several transportation-related tables to their planned set of ACS tabulations.

• This includes– Residence tabulations of households

such as: household size * # of vehicles– A FEW tables by Place of Work

More positive things about ACS

• The ACS will provide reliable data for large geographic units on an annual basis.

• The flip side is that the data for small areas will be compromised, with much larger standard errors, and the risk of the inability to tabulate small area (TAZ) home-to-work flows.

Questions for today’s discussion

• Do you want home-to-work flow (Part 3) from ACS if the geography is limited to tract-to-tract, or “tract group”- to- “tract group”

• Should AASHTO to sponsor a pooled fund for a CTPP from ACS?

AASHTO pooled fund

• Special tabs for large geography completed annually, starting perhaps in 2007? 2008?

• Special tab for “small geography”, conducted every 5 years, starting with the accumulated records of 2005 thru 2009, OR accumulated records of 2008-2012.

Transportation-sponsored research on ACS

• 1995 meeting (1996 BTS publication) on Continuous Measurement

• NCHRP project for Guidebook on using ACS data.

• FHWA sponsored research at CB RDCs and Suitland using ACS microdata.

• TRB Conference for May 2005.

Findings• Seasonality does not seem to be a problem

for JTW variables (Hampden County only)• Loss in # of O/D pairs is significant, and

may cause problems for modeling applications (Westat, NCHRP)

• Tract level data by mode of transport seems reasonable by residence geography (chart next slide)

0

500

1,000

1,500

2,000

2,500

3,000A

CS

-000

200

CTP

P-0

0020

0

AC

S-0

0520

0C

TPP

-005

200

AC

S-0

0670

0

CTP

P-0

0670

0A

CS

-008

500

CTP

P-0

0850

0A

CS

-011

501

CTP

P-0

1150

1A

CS

-013

200

CTP

P-0

1320

0

AC

S-0

1540

0C

TPP

-015

400

AC

S-0

1750

0C

TPP

-017

500

AC

S-0

2010

0C

TPP

-020

100

AC

S-0

2150

2

CTP

P-0

2150

2A

CS

-022

702

CTP

P-0

2270

2A

CS

-025

100

CTP

P-0

2510

0

AC

S-0

2890

0C

TPP

-028

900

AC

S-0

3180

0C

TPP

-031

800

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0C

TPP

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AC

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3610

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CTP

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-037

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CTP

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-038

600

CTP

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3860

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902

CTP

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3990

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AC

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4140

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Tract Number

Num

ber o

f Wor

kers

HighEstimateLow

Bronx County, Streetcar, trolley car, subway, or elevated

Key issues for Transportation

• Sample size / geographic level of reporting / data quality / error

• Thresholds for FLOW tabulation• CB Disclosure Review Board• How to do Trend Analysis?

Sample Size

• Long Form 16.6% of addresses • ACS (60 months) 12.5% of addresses• HOWEVER, that is the sample FRAME,

not the number of completed returns. Because the Mail-back response rate is much lower for ACS, even with 1:3 NRFU, the # of records in ACS is significantly lower than LF.

Ratio of ACS/LF unweighted person records

Broward* Multnomah Bronx*

ACS sampling rate (3 yr total)

10% 15% 10%

Unweighted LF person records

183,840 90,320 149,310

Unweighted ACS person records

94,288 64,445 57,069

Ratio ACS/LF 0.51 0.71 0.38

Implications of smaller number of completed surveys

• Kiss TAZ data GOODBYE.• Need a geographic unit maybe a “Tract

Group” or a TAZ that is as large as 2 or 3 tracts together, especially for home-to-work flow data.

Thresholds required for CTPP2000 for flow tabulation

• Thresholds are based on unweighted records.

• The smaller number of completed records in ACS results in a loss of O/D pairs meeting the threshold.

Impact of Thresholds Comparing Decennial LF to ACS Part 3

Broward County, FL Tract-to-TractPt 3 WITHOUT Thresholds

Pt 3 WITH Thresholds Part 1 total workers

# of Pairs Total Workers

# of Pairs Total Workers

LF 36,300 777,900 8,200 424,700 742,600

ACS 21,500 616,500 2,800 237,200 747,400

8% of O/D Pairs30% of workers

22% of O/D pairs

Does 60 months of accumulation result in reduced risk of

disclosure? We think “yes”• People move residence• People move workplace location• People change means of transportation to

work.• People change departure time.• People change # of vehicles in hhld.

Therefore…

• The Census Bureau should not require as much rounding or require thresholds for tabulation!

What rules will the DRB impose?What thresholds will the DRB impose for

flow tabulation? – We hope NONE! – If the threshold is 3 for key tables, as in 2000,

will transportation planners find it useful or not?

Biggest threat

• The combination of smaller samples and potential requirements of thresholds (unweighted records) for tabulation by DRB could result in a CTPP that is primarily used for residence tabulations, with limited workplace tabulations, and flow data limited to total worker counts.

What do we want from the CB?

• Alternatives to rounding and thresholds for disclosure avoidance.

• Improved allocation procedures. Want to continue working with CB on allocation and imputation of means of transportation, # of vehicles, and place of work geocoding.

How to use TREND data?Example: Tulare County, CA

Tulare County, CA

2000 Census

2000 ACS 2003 ACS

Median hhld income

$33,983 $31,467 $36,343

%hhlds with 0 vehicles

9.7% 8.1% 7.8%

%workers carpooling

18.7% 14.0% 12.6%

How to use TREND data?Example: No 2000 ACS data available

County with population below 250,000

2000 Census

2000 ACS 2007 ACS

Median hhld income

$34,000 n/a $38,000

%hhlds with 0 vehicles

10.0% N/a 8.0%

%workers carpooling

18.0% N/a 12.0%

How should we augment ACS data?

• Explore alternatives for Home-to-Work Flow matrices, including LED

• Consider workplace surveys• Add group quarters surveys, unless ACS

implements as planned.• NHTS for 2008, for trip length distribution

curves for various trip purposes/activities

Questions for today’s discussion

• Do you want home-to-work flow (Part 3) from ACS if the geography is limited to tract-to-tract, or “tract group”- to- “tract group”

• Should AASHTO to sponsor a pooled fund for a CTPP from ACS?

• Please come to the TRB conference in May!

Thank you for your attention.