1. Emerging Issues for Transportation Data Users of ACS Elaine Murakami, FHWA Elaine.murakami@fhwa.dot.gov TRB Planning Applications Conference, April 26, 2005

2. 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

3. 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.

4. 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?

5. 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.

6. 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.

7. 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)

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

9. 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?

10. 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.

11. Ratio of ACS/LF unweighted person records Broward*MultnomahBronx* ACS sampling rate (3 yr total) 10%15%10% Unweighted LF person records 183,84090,320149,310 Unweighted ACS person records 94,28864,44557,069 Ratio ACS/LF0.510.710.38

12. 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.

13. 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.

14. Impact of Thresholds Comparing Decennial LF to ACS Part 3 Broward County, FL Tract-to-Tract Pt 3 WITHOUT Thresholds Pt 3 WITH ThresholdsPart 1 total workers # of PairsTotal Workers # of PairsTotal Workers LF36,300777,9008,200424,700742,600 ACS21,500616,5002,800237,200747,400 8% of O/D Pairs 30% of workers 22% of O/D pairs

15. 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.

16. Therefore… The Census Bureau should not require as much rounding or require thresholds for tabulation!

17. 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?

18. 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.

19. 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.

20. How to use TREND data? Example: Tulare County, CA Tulare County, CA 2000 Census 2000 ACS2003 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%

21. How to use TREND data? Example: No 2000 ACS data available County with population below 250,000 2000 Census 2000 ACS2007 ACS Median hhld income $34,000 n/a$38,000 %hhlds with 0 vehicles 10.0%N/a8.0% %workers carpooling 18.0%N/a12.0%

22. 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

23. 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?

24. Please come to the TRB conference in May! Thank you for your attention.