HERS-ST Highway Economic Requirements System - State Version: Technical Report - Chapter 1: Introduction
The Highway Economic Requirements System-State Version (HERS-ST) is a software package that estimates the investment that would be required to achieve certain highway system performance levels. Alternatively, the software can be used to estimate the highway system performance that would result given various investment levels. HERS-ST considers capital improvement projects directed at correcting pavement and capacity deficiencies.
The HERS-ST model is a direct extension of the national-level HERS model. This model was developed by the Federal Highway Administration (FHWA) to examine the relationship between national investment levels and the condition and performance of the Nation's highways. FHWA uses the model to estimate future highway investment requirements under different scenarios. FHWA provides this information to the U.S. Congress in the biennial Status of the Nation's Highways, Bridges and Transit: Conditions and Performance Report to Congress.
The HERS-ST model includes not only the analytical engines which comprise the national model but also a graphical user interface (GUI) module. The GUI environment includes tools for organizing the HERS-ST workspace, performing the HERS analysis, and organizing and formatting program output.
This Technical Report is part of a set of HERS-ST documentation. It consists of detailed technical discussions of the procedures, assumptions, algorithms, and inputs of the HERS-ST model, focusing on the analytical engines. The other HERS-ST documents are:
The HERS-ST User's Guide provides "hands-on" assistance to the analyst interested in using HERS-ST to evaluate alternative highway program and policy scenarios. It contains detailed information and procedures for using the many tools provided in the GUI.
The HERS-ST Overview offers a summary of the model's logic structure and the analytical, economic, and engineering procedures it utilizes.
The HERS-ST Pilot Program Report summarizes the findings of the HERS-ST Pilot Program and discusses the steps taken to create the first version of HERS-ST intended for national distribution.
HERS-ST calculates the benefits resulting from potential highway improvements, quantifying three types of benefits to highway users (travel time, operating costs, and safety), benefits to highway agencies (reduced maintenance costs), and one "external" benefit (damage caused by vehicle emissions). The model also calculates the "residual value" of improvements with remaining service life at the end of the analysis period. The HERS-ST model uses benefit-cost analysis to evaluate potential improvements when selecting among candidate improvements for implementation. As part of its benefit calculations, HERS-ST captures the effect of improvements on traffic levels and the associated changes in pavement condition, pavement life, and user costs. Finally, the results of comparing alternatives are distilled into summary measures of benefits and costs for candidate improvements.
The remaining seven chapters of this Technical Report can be briefly summarized as follows:
Describes the structure of the HERS-ST system and indicates generally how the model components are interrelated and the time and other dimensions over which the model operates.
Explains the different types of HERS-ST inputs, the processing performed by the HERS-ST PreProcessor, and several underlying HERS-ST assumptions.
Defines the different types of improvements that HERS-ST can implement to correct deficiencies identified for a section, and the impacts of these improvements on the highway and the user.
Describes the way the HERS-ST models discrete processes, including: pavement wear, vehicle operating costs, crash costs, traffic forecasts, speed calculation, travel time costs, demand elasticity, agency costs, and external costs.
Describes the initial costs of improvements used by HERS-ST, procedures used for estimating these costs, the model's methodology for adjusting costs from year to year, and the different state cost factors.
Explaining the mechanics of how HERS-ST quantifies and discounts benefits, and evaluates projects using benefit-cost criteria.
Describes the output that is automatically generated by HERS-ST.
The Technical Report also includes six appendices providing additional background and detail about HERS-ST:
Provides the suggested default values for the three different types of section deficiency criteria. Also explains the role of default design standards and user specified thresholds.
Describes the concepts guiding several modifications that were made to the HERS-ST model to account for traffic effects.
Describes the procedure for establishing the values that represent the short-run generalized price elasticity of travel, and the estimation of the long-run share parameter.
Discusses the principles that have served as a guide in developing the economic portions of the HERS-ST model, and how the model embodies them.
Describes the disaggregated vehicle operating costs by vehicle type and component for both constant and variable speeds. Also explains the pavement condition adjustment factors for the constant speed operating costs.
Discusses the effect that traffic volume, the mix of vehicle types, and average effective speed can have on air pollution, and how HERS-ST attempts to measure these effects.