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HERS-ST Highway Economic Requirements System - State Version: User's Guide

Glossary and Key Concepts

This section expands on common HERS-ST terms and concepts identified earlier in this document, but provides greater detail to facilitate your better understanding of the software.

Advanced Features
Advanced Features permit the user to access the enhanced capabilities of the application, which include the ability to edit all portions of the HERS-ST control and parameter settings. You enable the Advanced Features through Tools/Options. Also see: Standard Features.
Analysis Software

Analysis is the resolution of problems by reducing the conditions that are in them to equations. The HERS-ST is itself analysis software as it attempts to optimize the relationship between public highway investment and user costs. The HERS-ST model simulates highway condition and performance levels and identifies deficiencies through the use of engineering principles, but it relies upon economic criteria to select improvements for implementation.

HERS-ST is designed to select only those improvements where benefits will exceed initial costs. An improvement's benefits consist of reductions in user costs, agency maintenance costs, and externalities over the life of the improvement. Costs consist of the initial capital costs of the improvement. HERS-ST attempts to optimize the relationship between public highway investment and user costs.

Analysis Type
The mechanism by which HERS-ST selects highway section improvements to implement, HERS-ST offers the four types of analysis: Full Engineering Needs, Minimum BCR, Maximum Benefits Constrained by Funds, Minimum Cost Constrained by Performance.
Basic Mode
In basic mode, HERS-ST evaluates the costs and benefits of all potential pavement, widening, and alignment improvements and determines which improvements best meet the criteria you provided. Information about the state of the highway system in the base year and forecast traffic volumes for some specified future year is provided by the HPMS sample section data recorded in the Highway data. When running HERS-ST software in Basic mode, the State Improvements data is ignored. Also see: Override Mode.
Benefit Cost Ratio (BCR)
Usually, the Benefit Cost Ratio (BCR) of a highway improvement is the discounted sum of the present values of the user, agency, and environmental benefits divided by the implementation costs of the improvement. An exception occurs when the system is operating under a funding constraint or with a performance goal and two or more alternative improvements are provisionally selected for a section in sequence. In this case, the BCR of the improvement is an incremental BCR; i.e., it is derived from the incremental benefits and costs of choosing the improvement that is finally selected to replace the last previous improvement that had been provisionally selected.
The clone capability allows you to create a copy, or clone, of an object and display it in the Workset tree. The clone is identical to the source object except an index is appended to its name to indicate its status.
Continuation Analysis
This is one of two types of multiple-run analysis. At the end of each analysis run, the state of the highway system is preserved and used as the starting point for the next run. You can select a Control and Parameter model to be used for each run iteration.
Constrained by Funds

This is one of four HERS-ST analysis types with the goal to maximize the net present value of the benefits of improvements as subject to user specified constraints on funds available during each funding period. The constrained-by-funds analysis addresses questions such as the following:

  • What level of system condition and performance can be obtained when the improvements are limited by the funds you specify?

Also See Chapter 3.

Constrained by Performance

This is one of four HERS-ST analysis types with the goal to maximize the net present value of the benefits of improvements as subject to user specified constraints on system performance during each funding period. The constrained-by-performance analysis addresses this question:

  • How much will it cost to achieve a specified level of system performance?

Also See Chapter 3.

Control Settings

The Control settings provide the settings for the HERS-ST analysis software. These settings specify performance, cost, weight, traffic, and other analytical factors. A Workset may have any number of control settings; however, only one can be active at a time. The control settings are made up of two items:


Counterpeak lanes are the number of lanes in the "non-peak" travel direction during a peak travel time period. This value is usually obtained by subtracting the number of peak period lanes in the peak direction from the total number of through lanes. However, for some sections, HERS-ST infers the likely existence of extra peak-period lanes resulting from parking restrictions or use of shoulders as travel lanes during the peak period. For these sections, total peak lanes will exceed off-peak through lanes. For rural sections with fewer than four lanes, peak lanes represent the total number of lanes in both directions, and counterpeak lanes equals zero.

Corresponding to peak, counterpeak, and through lanes are three values of capacity: peak, counterpeak, and off-peak capacity. For rural roads with fewer than four lanes, the peak and off-peak capacities are two-way capacity, and counterpeak capacity is zero. For all other roads, the three capacities are one-way capacities. Peak capacity is the capacity coded in the HPMS record. Off-peak and counterpeak capacities may be equal to or smaller than peak capacity.

Default Cost Data

All cost data defaults are derived from national averages, including unit costs for highway improvements, vehicle operations, travel times, injuries, and property damage, as well as fuel excise taxes and the value of life. The costs are supplied in dollars of various years and are converted to base year 2004 dollars using price index values. Estimates can be produced in any other year dollars by appropriately adjusting these price indexes.

The FHWA's estimates of national unit costs for highway improvements have been adjusted to base year 2004 dollars using the FHWA's composite price index for federal-aid highway construction. The system then adjusts these costs to for each individual highway section using the Local Cost Factor (LCR) field in the highway data. State users may wish to incorporate their own estimates of pavement and widening costs into the system.

Deficiency Level
Deficiency levels are settings that determine the condition threshold when a section is considered deficient and in need of improvement. The HERS-ST deficiency levels model is one of three components that make up the Parameter Settings.
Design Period
The design period represents the expected life of pavement in years. This time frame is used in estimating future traffic volume for the purpose of determining future lane (capacity) requirements. HERS-ST uses the Average Annual Daily Traffic (AADT) to model traffic growth from the year an improvement is implemented through the end of the design period.

Elasticity measures the responsiveness of an analytical output to changes in input values. HERS-ST focuses on the elasticity of traffic demand to changes in the cost of travel. Specifically, HERS-ST uses two parameters to account for induced demand; a short-run elasticity (SRE) and a long-run share of elasticity (LRS). The concept of induced demand and its representation in HERS-ST are discussed in the paper titled HERS-ST Induced Demand and Elasticity.

Short Run Elasticity (SRE). The value for this parameter represents the short-run responsiveness of traffic volume to changes in the user price of travel within a funding period.

Long Run Elasticity (LRE). The value for this parameter represents the long-run responsiveness of traffic volume to changes in the user price of travel over a longer (typically a 20 year) period.

Response to the SRE is assumed to take place immediately whenever a price change occurs within a funding period. The long-run effect is represented by a shift in the demand curve between funding periods.

Long Run Share (LRS). Because HERS-ST funding periods are finite in length and less than 20 years, HERS-ST never reaches a true long-run equilibrium. Thus the purely long-run component cannot be implemented directly. Instead, an elasticity that shifts the demand curve between funding periods is used called the Long Run Share.

The LRS is dependent on the value of the SRE, LRE and the length of the funding period. For short periods, the LRS is small in magnitude and for longer periods it is larger. The longer the funding period, the relatively greater will be the shift in the demand curve between periods. The LRS is calculated from the SRE and LRE using a simple model of how rapidly the market approaches equilibrium.

Functional Class

All public roads and streets in the United States are assigned a functional classification by their type and use. HERS-ST utilizes the following nine functional classes of the overall highway system functional classification hierarchy:

  • Urban Collectors
  • Urban Minor Arterials
  • Urban Other Principal Arterials
  • Urban Principal Freeways/Expressways
  • Urban Principal Interstates
  • Rural Major Collectors
  • Rural Minor Arterials
  • Rural Other Principal Arterials
  • Rural Principal Interstates

Analysis results are reported by funding period and by functional class. Within HERS-ST, there are several groupings of highway types by functional class. Sometimes these groupings are also disaggregated by AADT range and/or other attributes. Within HERS-ST, Urban Locals are grouped with Urban Collectors, and Rural Minor Collectors and Rural Locals are grouped with Rural Major Collectors.

Funding Period

The HERS model operates in a simplified discrete time frame, or funding period, in which events occur at points in time that are determined by the length of the funding period. Results are summarized and reported by funding periods, but the analysis itself is oriented to Benefit Cost Analysis (BCA) periods, and subdivisions of the BCA period that are called demand periods. The length of a demand period is the same as a funding period, but goes from the midpoint of one funding period to the midpoint of the next funding period.

The analyst determines the length of the overall analysis period and the length of the funding periods. Typically, a HERS-ST evaluation is conducted over a 20-year horizon divided into four funding periods of five years each. HERS-ST will implement no more than one improvement per deficient highway section for each funding period.

The funding period is a simplification of the real world, in that decisions are assumed to occur once within a funding period and all projects are implemented instantaneously. The midpoint of the funding period represents the period as a single point in time. The FP length can be potentially varied between one and seven years, with shorter periods taking longer computational times.

Geographical Information System (GIS) Map
The GIS map object is used as a reporting mechanism to create maps of the highway sections that need to be improved. GIS maps can be used to show the distribution of traffic flow, pavement conditions, or any other information collected or generated by the HERS-ST software.
Highway Performance Monitoring System (HPMS) File
The Highway Performance Monitoring System (HPMS) sample section data file contains ASCII text descriptions of each highway section to be analyzed, one record (or row of text) per highway section, in the standard HPMS comma-delimited format. Each record contains each of the 98 HPMS Data Fields. In general, each section record must be complete with all data items in order for HERS-ST to perform a project evaluation on that section. For some missing data items, however, HERS-ST can attempt to estimate the value using other data items in the same record and applicable models.
Improvement Costs
Improvement costs contain the cost estimates of improvement activities used by HERS-ST to determine economic feasibility of an improvement. The HERS-ST improvement costs model is one of three components that make up the Parameter Settings.
Maintain Current Conditions Analysis
The Maintain Current Conditions analysis is a form of the Performance-constrained analysis in which the current level of system performance is the performance goal.
Minimum BCR Analysis

This is one of four HERS-ST analysis types with the goal to implement all improvements with incremental benefit-cost ratios greater than a user-defined threshold value. The Minimum BCR Analysis addresses the following questions:

  • Which improvements exceed a specified minimum benefit cost ratio?
  • How much can be invested at this level of return?
  • What will be the condition and performance of the highway system after investing at this level?

Also See Chapter 3.

Multiple Minimum BCR Run
The multiple minimum BCR run is a special type of sensitivity analysis that conducts one or more Minimum BCR Analysis runs while incrementing the minimum BCR value between each run. You must specify the starting, ending, and increment value to be assigned to the minimum BCR for each run. The number of runs conducted is based upon the size of the minimum BCR range and the value of the increment.
Outputs Settings
The Output settings are used to control what HERS-ST analysis data is captured by the database and is available to the user. You can use these settings to reduce the amount of data stored in the database and to parse it to focus on just the areas of interest you need.
Override Mode

The Override mode capability provides you with the ability to override improvement recommendations produced by HERS-ST with user-specified improvements provided in the State Improvements model.

In the State Improvements model, you can specify which improvements are to be made on specified sections as well as when these improvements are to be made, the initial costs of these improvements, and their effects on capacity. You can also use override mode to specify improvements that were not selected in the basic mode. See also: Basic Mode.

Parameter Settings

The Parameter settings provide settings for the HERS-ST analysis software. A Workset may have any number of parameter settings; however only one can be active at a time. The parameter settings are made up of three items:

Parameters contain many general settings for the HERS-ST analysis software. The HERS-ST parameters model is one of three components that make up the Parameter Settings
Pavement Condition Measure
HERS-ST uses the present serviceability rating (PSR) as an indicator of pavement condition. All internal calculations such as speed, operating costs, pavement deterioration, and improvement identification are based upon the PSR values provided for each highway section in the Highway Data file. HERS-ST obtains the initial value of PSR from the value of IRI coded in the HPMS record. If IRI is not coded, HERS-ST uses the coded value of PSR instead.
Pre-Processor Specification
The pre-processor specification (PPSpec) model is one of two components that make up the Control Settings. It contains the pre-processor control settings for the HERS-ST analysis software.
Predefined Charts
Predefined charts allow you to quickly assemble commonly used charts from the analysis results. They are divided into two categories. The first category consists of charts that are applicable to a single analysis run; the second consists of charts that are applicable to multiple runs.
Query Builder
The Query Builder provides the ability to create a query in structured query language (SQL). Using the Query Builder tool, you can build complex criteria conditions with which to select highway records in the Highway Data Sections grid and to retrieve data from the results of any analysis scenario for use in charts, reports, and tables.
Reports, which are displayed in the third tab in the Workset Viewer, refer to the user-defined charts, GIS maps, tables, and ad-hoc reports created based on the data stored in the underlying database.
Results refer to the data generated by the analysis process and stored in the HERS-ST's underlying database. Results include Section Conditions, System Conditions and Improvement Cost by Functional Class.
A grouping mechanism used within a Scenario to organize the resultant output data associated with a specific run of the HERS-ST analysis. Every time an analysis is performed and a Scenario is created, one or many runs are added to the Scenario. Each run contains a copy of its Control and Parameter settings and the results of the analysis.
Run Specification
The run specification (RunSpec) model is one of two components that make up the Control Settings. It contains the control settings for the HERS-ST analysis software.
A grouping mechanism used within a Workset to organize the data associated with a specific run or group of runs of the HERS-ST analysis. Every time the analysis is performed, a Scenario is created and added to the current Workset. This Scenario will contain a read-only copy of all of the highway data and/or State improvements data used in the analysis.
A portion of the highway system as defined by the user. Highway sections are usually homogenous for lane width, surface type, age, traffic, and jurisdiction.
Section Conditions
The Section Conditions results contain information about the condition of each highway section at the beginning and end of each funding period. This information is generally used for creating GIS maps that show the conditions of the highway sections in a graphical form.
Sensitivity Analysis
This is one of two types of multiple-run analysis. This type of analysis is used to determine how sensitive the analysis is on a particular factor or parameter. After each analysis run, the highway system is returned to its original conditions. You can select a Control and Parameter model to be used for each run iteration.
Settings, which are displayed in the first tab in the Workset Viewer, refer to the input data and settings for the HERS-ST analysis.
Standard Features
Standard Features permit the user to access the standard capabilities of the application, the most commonly modified items in the HERS-ST control and parameter settings. Also see: Advanced Features.
Structured Query Language (SQL)
SQL (often pronounced "sequel") is both an interactive query language and a database programming language. It is an industry standard used by numerous relational database applications such as those from Oracle, Microsoft, and Sybase. For purposes of HERS-ST, the focus is strictly related to the interactive query capability of SQL. The query capability of SQL allows you to construct queries ("questions" about the data) using the SQL language. The results ("answers") of these queries are returned in the form of a Record Set, which can be defined as the delimited, tabular (rows and columns) results from the database that satisfy the criteria specified in the query. See industry documentation for advanced information on the use and syntax of SQL.
User-Specified Thresholds (USTs)

Users-Specified Thresholds (USTs) are limit points, set by the user, and used by HERS-ST for generating the Results Deficiency pages, which may be generated as part of the Tabular Output results. The Results Deficiency page presents information about the relative extent of various shortcomings in the highway system. The UST values have no effect on any other HERS-ST results.

As an example, a UST could be used to produce information about the percent of pavement that meets or fails one or more sets of state-specified standards for pavement condition. After performing the analysis against these thresholds, the deficiency information in the Improvement Statistics would show the percent of road miles that have IRIs that exceed various user-specified thresholds.

Data validation is a mechanism to ensure that input data are of appropriate quality to use in the analysis. There are many HPMS validation rules or conditions that can be applied to the highway sections. Sections that fail any of the rules are displayed with a red-tinted background in the highway data table.
Vehicle Type
The HERS-ST model divides traffic into seven types of vehicles. The full breakdown is not in the HPMS sample section data items; instead, the share of single-unit and combination trucks in the traffic stream is reported. In this definition, a truck is anything with six tires or more, which could be a two-axle vehicle but not a standard pickup truck. HERS-ST then applies aggregate national distributions derived from the HPMS Vehicle Classification Study to allocate single-unit and combination trucks into the seven subclasses. Thus the classifications are obtained directly from the section data record, whereas the seven vehicle types are expanded from the three classifications using average distributions by functional class. In this case, HERS-ST contains procedures to utilize a finer level of detail than is currently available from the HPMS.
Workset is the highest level of organization used within HERS-ST. In simple language, the Workset is the technical problem or analytical task that you are using HERS-ST to solve. The Workset grouping is used to include the input data and settings, analyses, results and user-defined reports associated with the analytical task. When a run is performed, the input data and settings contained within the Workset are selected and the results of the analysis are grouped into a Scenario. Any reports (charts, GIS maps, tables, or ad-hoc reports) that are created based on the data referenced by the open Workset are also stored as part of the Workset.
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Updated: 02/14/2013