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HERS-ST

Highway Economic Requirements System State Version (HERS-ST) was developed for the Federal Highway Administration as a optimization framework to help federal and state authorities, and others, to develop highway investment programs and policies that maximize economic benefits relative to costs. (HERS-ST is the version of the program designed for state and regional agencies.) The HERS family of models can be used to explore the relationship between levels of investment and the performance of the highway system. The program is closely linked with the Highway Performance Monitoring System (HPMS), a federal data system used to track the characteristics, usage, and condition of the nation's highway system.

Project Types Evaluated: HERS-ST uses information about the current highway system and its deficiencies, obtained from HPMS, and generates a candidate set of standard highway improvements, which may be augmented by additional user-specified improvements. It then searches for the best combination of improvements for which the economic benefits exceed the costs. Up to six different investment alternatives are considered for each highway segment by combining possible improvements to pavement, width, and alignment. Options to construct entirely new highway segments or to improve non-highway modes are not considered, nor are improvements to major bridges and tunnels.

Scope of Application: HERS-ST selects economically desirable improvements for nine major functional classes of highways, excluding local facilities and minor rural collectors. Candidate improvement projects are first identified for each segment of a highway system serving a state or region. Existing and forecasted future conditions are described, with and without the candidate improvements. The optimum combination of improvements is then determined.

Three different optimization scenarios are available:

  • Determine the most economically desirable combination of projects subject to a specified funding level.
  • Determine the minimum spending that satisfies specified performance targets, which are defined in terms of user or agency costs.
  • Determine the combination of projects and the level of spending needed to implement all candidate projects that meet a target economic criterion (e.g., a minimum B/C ratio, typically 1.0).

Candidate improvements for each highway segment in each funding period are determined based on user-defined deficiency criteria specified in terms of pavement conditions, surface types, congestion (v/c) levels, lane and shoulder widths, and alignment (curvature and grades). Additional user-specified improvements may be forced into the solution. The optimization is performed for a user-specified analysis period (typically 20 years), divided into multiple funding periods (for example, 5 years). Present and future Annual Average Daily Traffic (AADT) values plus K and D peaking factors from HPMS are used to characterize traffic. The program adjusts segment AADT's using an elasticity-based procedure that suppresses traffic under congested conditions and may predict induced traffic for alternatives where capacity is increased. Traffic adjustments consider only the conditions of the highway segment itself, since the program does not explicitly address diversion within the network from unimproved to improved segments.

Benefit Categories Considered:

  • Changes in user travel times
  • Changes in vehicle operating costs (fuel, oil, tires, maintenance, depreciation)
  • Changes in collisions
  • Changes in emissions (combined costs of CO, NOX, PM10, VOC, SOX, and road dust)
  • Changes in agency costs for highway maintenance and operations
  • Changes in highway residual values

Cost Categories Considered:

  • Initial right-of-way acquisition
  • Construction costs

Economic Performance Measures Provided:

  • Incremental benefit-cost ratios for selected improvements

Other Quantitative Impacts Considered:

  • Measures of congestion (peak volume-capacity ratio)
  • Speed by segment and averaged by functional class
  • Delays
  • Pavement condition (PSR and IRI) by segment and averaged by functional class
  • Selected geometric improvements for each highway segment
  • Deficiency ratings, before and after selected improvements
  • Collision rates, before and after selected improvements

The HERS software and documentation are available for free downloading at: http://www.fhwa.dot.gov/infrastructure/asstmgmt/hersindex.cfm.

Example

Due to the program's complexity, no sample problem is presented. Instead, the reader is directed to a collection of example screen images illustrating how HER-ST was used to evaluate a program of improvements for 1255 highway segments in an HPMS database. These are on-line at: http://www.fhwa.dot.gov/infrastructure/asstmgmt/hersrprep.cfm.

A HERS-ST analysis uses a "project data set" containing existing highway segments (in HPMS 2000 format); optional user-mandated improvements; one or more parameter models, which specify design standards, costs, and deficiency criteria; and one or more control models, which specify other program options. Following an analysis, the project data set also contains results in standard format, along with any user-defined output displays. Multiple copies of each data type may exist in the same project data set, which is formatted as an MS Access database (.mdb) file.

HERS-ST (Version 2.0) has a utility called the "Project Wizard" which leads the user through a step-by-step interview to create a new project data set, from scratch or by modifying existing project data. The wizard can import and validate HPMS highway data, define user-mandated segment improvements, and create the necessary parameter and control models.

HERS-ST expects the existing highway segment data to be imported directly from an HPMS database, although any appropriately formatted highway section data can be used. If a full standard HPMS database is input, portions of the data can be selected for the analysis, and other highway sections ignored. The highway data can be edited as desired.

Up to 10 different user-mandated improvements can be specified for each highway section. They can be defined by the user, chosen from a menu of 18 standard types or be a combination of both. Each standard type includes different combinations of pavement improvements, shoulder improvements, widening, and alignment improvements. Each user-specified improvement can be applied in a "take it or leave it" fashion, or used as a minimum level that HERS can improve upon if economically justified. Improvements are implemented in chronological order. Default improvement costs and the corresponding capacity increases are estimated internally for standard improvement types.

Each HERS parameter model contains a variety of engineering performance and cost estimates and standards for different types of pavement and geometric improvements. Deficiency levels that may trigger improvements are defined using measures of congestion (volume-capacity ratio), shoulder and lane widths, and pavement conditions, separately for different functional classes, terrain types, and daily traffic. Pavement conditions and the effects of possible pavement improvements are characterized by Present Serviceability Rating (PSR) thresholds, defined for rural and urban highway segments and for high, medium, and low type rigid and flexible pavements. Pavement conditions are also characterized by the International Roughness Index (IRI), which is converted internally to PSR. State-specific default unit costs ($/lane-mile) are provided for the nine standard types of pavement and widening improvements for urban and rural conditions, based on functional class and terrain type.

The user can define quantitative deficiency thresholds for each highway segment in eight categories:

  • Pavement condition
  • Surface type
  • Volume-capacity ratio (peak period congestion)
  • Lane width
  • Right shoulder width
  • Shoulder type
  • Horizontal alignment
  • Vertical alignment

Three thresholds are defined — "Deficiency," "Serious deficiency," and "Unacceptable," — reflecting the likelihood that improvements which correct these situations will be considered. Default thresholds are provided for different AADT's and functional classes.

HERS-ST incorporates a complex screening logic that examines the starting and forecasted conditions for each highway segment in each funding period, thereupon selecting a "short list" of as many as six alternative improvements, up to two of which are "aggressive" (relatively expensive), expected to cure most identified deficiencies, and up to four are "less aggressive" spot improvements. Costs and impacts are calculated for each segment with and without these improvements, and the best combination is found based on the selected optimization scenario.

HERS-ST incorporates several sophisticated models to estimate cost components of candidate segment improvements, vehicle operation, emissions, and maintenance costs, as well as to estimate other consequences such as vehicle speeds, pavement deterioration, induced traffic, and segment capacity. Model details are described in the HERS-ST Technical Report. (FHWA, 2002b)

An extensive library of national default unit cost data is provided, including costs of standard improvement types; vehicle operation; travelers' time; emissions; injuries, fatalities and property damage collisions; and fuel taxes. Default price indices within the model are used to convert cost data from their original years into year 2000 dollars. Emissions costs, which depend on speed and vehicle types, were developed using the Mobile6 and PART5 emission models, combined with damage costs from a U.C. Davis study. (McCubbin and Delucchi, 1996) All default data can be modified.

Each HERS control model contains parameters needed to control the optimization, identify key data files, and properly document the run. Depending on which of three optimization scenarios are chosen, certain other data are provided to constrain the optimum solution accordingly. In general, applicable funding and performance constraints are specified for different functional classes and funding periods. Weights may be specified for different cost components in certain optimization scenarios. The overall analysis time period, the funding periods, discount rate, and several other system-wide parameters are provided.

The basic optimization process considers all the improvement options specified for each highway segment in each time period and selects the combination of improvements that provides the best solution for the selected objective function, subject to constraints.

Candidate improvements are selected or rejected based on incremental benefit-cost ratios, calculated for the analysis time period as:

IBCR = ((UCB + ACB + ECB) - (UCI + ACI + ECI) + RV) / (ICI - ICB)

where:

  • UCB, ACB, ECB: total user costs (for travel time, vehicle operation, and collisions), agency costs (for maintenance) and external costs (for air pollution) for the base alternative which is being compared to an improvement alternative,
  • UCI, ACI, ECI: total user costs, agency costs, and external costs for the improvement alternative being considered,
  • RV: the residual value of the improvement at the end of the time period being considered, discounted back to the start of the period and treated as a benefit,
  • ICI, ICB: the initial investment costs for the improvement and the base alternatives, respectively.

For each highway segment, candidate improvements are evaluated in increasing order of their initial investment costs. Note that this is equivalent to selecting improvements based on net present worth.

The program is noteworthy for its ability to conveniently perform sensitivity analyses in order to test whether the solution of recommended investments is robust to changing system goals and underlying parameters. Note that HERS does not reallocate traffic to reflect highway improvements. However, traffic growth induced by improved capacity and operating conditions is included, with half the estimated user benefits counted for induced traffic, consistent with consumer surplus principles. (FHWA, 2002d)

HERS-ST has a powerful output-generation utility for organizing and presenting results in standard and user-defined tables, charts, and maps. Maps are generated in "shape" format, which is compatible with common GIS software. All project data files are formatted as MS Access databases to facilitate additional manipulation and post-processing.

Sources

HERS-ST v20: Highway Economic Requirements System - State Version Overview. FHWA-IF-02-057. Federal Highway Administration, Office of Asset Management. Washington DC. August 2002a. Available at: http://www.fhwa.dot.gov/infrastructure/asstmgmt/hersdoc.htm. Accessed March 2004.HERS-ST v20: Highway Economic Requirements System - State Version Technical Report. FHWA-IF-02-060. Federal Highway Administration, Office of Asset Management. Washington DC. August 2002b.Available at: http://www.fhwa.dot.gov/infrastructure/asstmgmt/hersdoc.htm. Accessed March 2004.HERS-ST v20: Highway Economic Requirements System - State Version User's Guide. FHWA-IF-02-059. Federal Highway Administration, Office of Asset Management. Washington DC. August 2002c. Available at: http://www.fhwa.dot.gov/infrastructure/asstmgmt/hersdoc.htm. Accessed March 2004.Lee, Douglass. HERS-ST v20: Highway Economic Requirements System - State Version: Induced Demand and Elasticity. FHWA-IF-02-055. Federal Highway Administration, Office of Asset Management. Washington DC. August 2002d. Available at: http://www.fhwa.dot.gov/infrastructure/asstmgmt/hersdoc.htm. Accessed March 2004.HERS-ST: Highway Economic Requirements System - State Version: Pilot Program Report. FHWA-IF-02-056. Federal Highway Administration, Office of Asset Management. Washington DC. August 2002e. Available at: http://www.fhwa.dot.gov/infrastructure/asstmgmt/hersdoc.htm. Accessed March 2004.

McCubbin, D. and M. Delucchi. "Health Effects of Motor Vehicle Air Pollution". Institute for Transportation Studies, University of California, Davis, 1996.