Analysis of Zone Sizes (or, are zones too big for useful analysis?)

1. Analysis of Zone Sizes(or, are zones too big for useful analysis?) Ashutosh Kumar, AECOM Consult David Schmitt, AICP, AECOM Consult 11th TRB Planning Applications Conference Daytona Beach, Florida May 6th-10th, 2007

2. Zone-Based Networks • Activity aggregated at zonal level • Key advantages • Reduces the level of analyses and computations • Reduce forecasting burden • Virtually all models rely heavily on zone-based networks • Microsimulation models still use them for destination choice, skims and assignment

3. Problems • Assumptions • Aggregation error • Homogeneity assumed where there isn’t any • Example on next slide…

4. Large Zone (Examples) Zone 503 (3+ mi2) 2000 Population – 7,700 2025 Population – 25,000 2000 Employment – 2,600 2025 Employment – 3,300 503 Zone 1129 (3+ mi2) 2000 Population – 13,900 2025 Population – 18,700 2000 Employment – 2,100 2025 Employment – 2,300 1129

5. 503 1129

6. Problems • Assumptions • Aggregation error • Homogeneity assumed where there isn’t any • Maintenance • Time consuming to update • Developed on base year conditions, not future year • Result: large amounts of activity in expansive zones • Consequences are troubling: • Highway modeling – leads to unrealistic (“overburdened”) assignments • Transit modeling – incomplete or inaccurate representation of transit access

7. How to Evaluate Zone Size? • No defined standards! • Began own analysis of zone size vs. activity • Developed straightforward technique comparing zone size against base & future year activity levels • Applied to nine models across the country • Welcome feedback on other models

8. Methodology • One record per zone: • Zonal area • Base year activity • Future year activity • Develop cumulative plot of the 3 fields • “Activity” is defined as: Activity = Population + 2*Employment

9. Example Plot

10. Plot Interpretation • ~50% of the zones are less than 0.5 mi2 • Only ~38% of total activity occurs in the smaller zones in the base year • Even less in the future year • ~62% of the activities occur in zones with area greater than 0.5 mi2 • Good candidate for zone splits

11. Denver

12. Grand Rapids

13. Southeast Florida (SERPM)

14. Jacksonville (JUATS)

15. Orlando (OUATS)

16. Atlanta

17. Columbus Ohio

18. Findings • Base year activity levels • 1 of 9 have 80% of their activity in zones < 1 mi2 • 7 of 9 have 60% of their activity in zones < 1 mi2 • 2 of 9 have 60% of their activity in zones < 0.5 mi2 • Future year activity levels • 0 of 9 have 80% of their activity in zones < 1 mi2 • 4 of 9 have 60% of their activity in zones < 1 mi2 • 2 of 9 have 60% of their activity in zones < 0.5 mi2 • Zone systems depend on regional density, regional growth & natural boundaries

19. Recommended Approach • Review the zones vs. the base year activities • Subdivide zones accordingly • Review the zones vs. the future year activities • Subdivide zones again! • Repeat the process for any major change in land use data

20. Thank you!

21. Additional slides

22. Charlotte