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Conditions and Performance

Status of the Nation's Highways, Bridges, and Transit:
2002 Conditions and Performance Report

Chapter 10: Sensitivity Analysis
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Executive Summary
Part I: Description of Current System
Ch1: The Role of Highways and Transit
Ch2: System and Use Characteristics
Ch3: System Conditions
Ch4: Operational Performance
Ch5: Safety Performance
Ch6: Finance

Part II: Investment Performance Analyses
Ch7: Capital Investment Requirements
Ch8: Comparison of Spending and Investment Requirements
Ch9: Impacts of Investment
Ch10: Sensitivity Analysis

Part III: Bridges
Ch11: Federal Bridge Program Status of the Nation's Bridges

Part IV: Special Topics
Ch12: National Security
Ch13: Highway Transportation in Society
Ch14: The Importance of Public Transportation
Ch15: Macroeconomic Benefits of Highway Investment
Ch16: Pricing
Ch17: Transportation Asset Management
Ch18: Travel Model Improvement Program
Ch19: Air Quality
Ch20: Federal Safety Initiatives
Ch21: Operations Strategies
Ch22: Freight

Part V: Supplemental Analyses of System Components
Ch23: Interstate System
Ch24: National Highway System
Ch25: NHS Freight Connectors
Ch26: Highway-Rail Grade Crossings
Ch27: Transit Systems on Federal Lands

Appendix A: Changes in Highway Investment Requirements Methodology
Appendix B: Bridge Investment/Performance Methodology
Appendix C: Transit Investment Condition and Investment Requirements Methodology
List of Contacts

Chapter 10 Table of Contents

  • Summary
  • Highway Sensitivity Analysis
    • Alternative Travel Growth Assumptions
    • Alternative Model Parameters
      • Elasticity Values
      • Value of Ordinary Travel Time
      • Value of Incident Delay Reduction
      • Value of a Statistical Life
      • Improvement Costs
      • Truck VMT Shares
      • Impacts of Alternative Parameters on the Cost to Maintain Highways and Bridges
  • Transit Sensitivity Analyses
    • Sensitivity Changes in PMT
    • Sensitivity to a 25 Percent increase in Capital Costs
    • Impact of Change in the Value of Time


This chapter explores the effects of varying some of the assumptions that were used to develop the investment requirement projections in Chapter 7. In any modeling effort, evaluating the validity of the underlying assumptions is critical. The results produced by the Highway Economic Requirements System (HERS) and the Transit Economic Requirements Model (TERM) are strongly affected by the values they are supplied for certain key variables. This chapter was first added to the 1999 C&P report to open up more of the modeling process and to make the report more useful for supplementary analysis efforts.

There is a great deal of uncertainty about the appropriate values for the 20-year travel growth rates on which HERS and TERM rely. The highway and transit sections both show the impact that changing these assumptions would have on the investment requirement projections. Alternative estimates of highway investment requirements are shown for scenarios in which future highway travel growth rates match those observed over the last 20 years and for scenarios in which travel growth is substantially lower than that projected in the Highway Performance Monitoring System (HPMS) sample data. The sensitivity of the estimated transit investment requirements to the growth rate forecast is analyzed by allowing three alternative growth rate inputs: 50 percent higher than the forecast, 50 percent below the forecast, and 100 percent below the forecast (i.e., zero transit passenger mile growth).

The chapter also includes other sensitivity analyses showing the impact of using alternative values for certain key model parameters (whose estimated values may be subject to some uncertainty). Both the highway and transit sections analyze the impact of increasing the unit improvement costs in HERS and TERM by 25 percent and the effects of variations in the value of time. The highway section also considers alternative values for additional parameters, including the value of a statistical life, truck shares, and travel demand elasticity.

Page last modified on November 7, 2014.
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