11th National Planning Applications Conference

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11th National Planning Applications Conference

• Topic Statewide Modeling Validation Measures and
  Issues
• Authors Dave Powers, Anne Reyner, Tom Williams,
  Paul Hershkowitz, Rob Bostrom
• Date May 9, 2007

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Modeling Issues

• Statewide Modeling Objectives
• Forecast major intercity volumes and roadway
  deficiencies
• Estimate urban external-internal and through
  volumes
• Freight transport growth and potential for
  intermodal diversion
• Evaluate major travel corridor improvements
• Urban bypass
• Intercity corridor

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Modeling Issues

• Off- Model Traffic
• Urban intrazonal / local private vehicle
• Rural local private vehicle
• Freight intra-county
• Commercial vehicles

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Modeling Issues

• Observed Traffic Counts
• Primary independent validation criterion
• Vehicle class not universally available
• Estimated traffic volumes
• Geographically extrapolated traffic volumes

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Modeling Issues

• Travel Demand Survey Data - Person
• National Samples not necessarily representative
  at the state level
• Not comparable to traffic counts
• Add-on samples can expand usefulness
• Only source of intercity trips

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Modeling Issues

• Travel Demand Inventory - Freight
• Inter-county, large vehicle only
• Requires extensive review for reasonableness
• Requires conversion of tonnage to vehicles
• Requires estimation of empty vehicle movements
• Comparison to counts problematic

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Modeling Issues

• Data Checking Traffic Counts
• Focus on rural higher functional classes
• Eliminate urban influence
• Review spatial and temporal consistency
• Eliminate inconsistent and/or estimated/transferre
  d counts

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2003 vs 2006 Counts
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Modeling Issues

• Data Checking Person Travel Surveys
• Review sample size by household categories
• Compare trip rates, distribution characteristics
  across household categories
• Supplement with similar state data if necessary

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Modeling Issues

• Data Checking Freight Flow Inventory
• Compare flow origins/destinations with
  development inventories
• If available, compare multiple year data
• Major flow desire lines - review with
  knowledgeable state staff
• Review load factors
• Review implied empty backhaul factors
• Assign and compare VMT and traffic on major
  roadways

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Modeling Issues

• Validation Reasonableness Tests Person Travel
• Statistical correlation of model dependent and
  independent variables
• Household trip rates
• Proportion of trips by purpose / length
• Average trip length / trip length distribution
• Compared against model development data

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Modeling Issues

• Validation Objective Tests Person Travel
• VMT by major functional roadway class
• Volumes across major rural screenlines
• Volumes at major state boundary crossings
• Volumes along Interstate and other major roadways
• RMSE and individual count deviation analysis
• Identification and explanation of outliers

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Validation Screenlines - Mississippi
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Validation Screenlines - Kentucky
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Modeling Issues

• Validation Reasonableness TestsFreight
• Statistical correlation of model dependent and
  independent variables
• Tonnage by commodity type, and internal/external
  orientation
• Replication of special generator
  origins/destinations
• Inter-district flows
• Average trip length / trip length distribution
• Load factors / empty backhaul rates
• Compared against model development data

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Modeling Issues

• Validation Objective Tests Freight
• VMT by major functional roadway class
• Volumes across major rural screenlines
• Volumes at major state boundary crossings
• Volumes along Interstate and other major roadways
• RMSE and individual count deviation analysis
• Identification and explanation of outliers

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Percent Deviation for Validation Links -
Mississippi
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Percent Deviation for Validation Links - Kentucky
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Modeling Issues

• Validation Expectations
• Recognize limitations of source data
• Focus on rural areas / higher functional classes
• Quantitative comparisons will be less accurate
  than experienced with urban models
• Validation quality may vary depending on model
  complexity, state development/road system
  complexity and traffic counting system

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Modeling Issues

• Lessons Learned
• Spend time and effort to fully understand
  assumptions and estimation procedures present in
  traffic counts
• Spend time and effort to get input person and
  freight data as free from logical inconsistencies
  as possible
• Focus model calibration and objective validation
  tests on facility types/volume groups where a
  statewide model can be reasonably expected to
  perform accurately