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This presentation discusses the impact of large zone sizes on transportation analysis, identifies problems and consequences, and proposes a methodology for evaluating and potentially subdividing zones to improve accuracy. By analyzing base and future activities within different zones, recommendations for zone size adjustments are provided to enhance modeling outcomes.
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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
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
Problems • Assumptions • Aggregation error • Homogeneity assumed where there isn’t any • Example on next slide…
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
503 1129
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
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
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
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
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
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