HERS-ST Highway Economic Requirements System - State Version: User's Guide
Chapter 2 HERS-ST Overview
HERS-ST estimates the investment required to achieve certain highway system performance levels. Within the HERS model, you specify input data, perform an analysis, and then generate your analysis results. With the information produced from your analysis results, you can then generate reports using four different types of document formats. You can select one of several analytical scenarios provided by HERS-ST and then tailor it by selecting from an array of values and parameters defined by you.
The analytical procedure relies on a database of records in the Highway Performance Monitoring System (HPMS) sample data format. This database supplies information regarding the highway system -particularly, its current condition and performance. The analytical procedure involves identifying highway deficiencies and candidate improvements based on engineering standards. Finally, the analytical procedure selects the most economically worthwhile improvement projects according to economic criteria and scenario specifications provided by you. Figure 2-1 provides an overview of the HERS-ST process.
Figure 2-1. Overview of HERS-ST Process.
The HERS-ST application allows for the evaluation of three general types of scenarios, which can be used for answering the specific questions:
- What level of spending is required to achieve an economically optimal program structure that implements all economically worthwhile projects?
- What user cost/condition/performance level will result from a given spending level?
- What level of spending is required to achieve a certain user cost level? The general scenarios may be tailored by providing various input values such as the discount rate and deficiency levels. You can specify the overall length of the analysis period and the length of the funding periods. In addition, you can obtain other information by creatively manipulating the HERS-ST analytical procedures.
In preparation for a HERS-ST analysis, you can specify up to six types of input data. As illustrated in Figure 2-1, three of these types are considered "internal" and four "external." The internal settings are part of the HERS-ST software and you need not do anything to these settings to run the analysis. A default value is provided for each internal setting. However, you are required to provide the highway data to perform the HERS-ST analysis. Each record of the highway data should include information specific to a particular highway section such as pavement condition, traffic volume, and vehicle mix. You may run HERS-ST for your entire highway system by creating a file for all highway sections with the same data items and format as your HPMS submittal file.
You can change the following internal settings or can keep the default settings:
Control Settings. Control settings contain information used to govern the HERS-ST analytical procedures. Setting the Control data enables you to direct how the analysis is to run by specifying such factors as the analysis objective and method.
Output Settings. Output settings are used to govern what information from the HERS-ST analysis is captured. You can use these optional settings to focus the data captured by HERS-ST into need-specific subsets of information.
You select the following input data from external sources:
Highway Data. In general, the starting point for the HERS-ST analysis is a Highway Data file containing a set of highway sections for a base year. This can be your State's HPMS dataset or State highway inventory data in HPMS sample section data format.
Improvement Data. You also can choose to specify the State improvements data you want HERS-ST to use during the analysis. This optional, user-defined information supplements the highway data and is used to override the HERS-ST recommended improvements on any given highway section.
Intermediate Year Data. You can provide an external file that defines significant changes to the highway system that occur during the time frame within a funding period. This optional data is useful to provide information, such as the effect on Average Annual Daily Traffic (AADT) from the opening of a new road, that is not provided in the highway data.
GIS Map Data. By specifying a GIS shape file that corresponds with your highway data as part of your input data, you can display a graphical image of the highway data sections you are including in the analysis. Using the graphical image displayed, you can view those highway sections that you have selected graphically and select highway sections for analysis.
The HERS-ST analytical procedures begin with an evaluation of the current state of the highway system based on the highway data you select. Using the initial section-specific traffic growth forecasts, HERS-ST projects future conditions and performance for each funding period (FP) through the end of the overall analysis period. A funding period is a user-specified period of time of three to seven years, representing a subset of the overall analysis period, for which HERS-ST analyzes, summarizes, and reports results. The default HERS-ST settings cover an overall analysis period of 20 years divided into four FPs of five years each.
At the end of any given FP, HERS-ST examines each highway segment, according to accepted engineering standards, and checks for deficiencies (e.g., poor pavement condition and high volume-to-capacity ratios). Then, using standard engineering practices, a set of potential improvement options is identified to correct the deficiencies. A benefit-cost ratio (BCR) is calculated for each potential improvement and the best improvement option is selected for each section. Usually, the BCR of a highway improvement is the discounted present value (at the time of the improvement) of the sum of the user, agency, and external benefits of the improvement divided by the implementation costs. The section improvements are prioritized based upon BCR and the most cost-beneficial improvement options are selected for system-wide implementation, given the analysis type you selected. Figure 2-2 illustrates this process.
Figure 2-2. HERS-ST Analytical Procedures.
The highway data you select provides HERS-ST with a "picture" of the current condition of the highway system. Each record contains information on a section's characteristics such as pavement type and condition, geometry, traffic volume, and traffic capacity. This information is the starting point for the HERS-ST analysis.
The highway data also include base year traffic volumes and forecasts of future traffic levels (at a specified date) for each highway segment. This baseline travel forecast is then adjusted based on anticipated changes in travel demand on the segment in response to reductions or increases in average highway user costs as highway improvements are selected or rejected.
Future Conditions and Performance
The traffic growth information is used to forecast traffic, volume-to-capacity (V/C) ratios, and pavement condition for each FP in the overall analysis period. HERS-ST uses this information to estimate the point at which a pavement or capacity improvement will be required and the extent of improvement indicated. The methods for forecasting V/C ratios and pavement condition are based on standard engineering practices, pavement deterioration models, and capacity calculations. As appropriate, the section data are revised, in each FP, to reflect the simulated changes expected to occur over time due to either the implementation of an improvement or continued deterioration in the absence of an improvement.
At the end of each FP, HERS-ST checks the following highway elements for deficiencies based on criteria you supplied: pavement condition, V/C ratio, surface type, lane width, right shoulder width, shoulder type, horizontal alignment (curves), and vertical alignment (grades). HERS-ST allows you to determine three different deficiency levels - normal, serious, or unacceptable - for each of the highway elements of interest.
Potential Improvement Identification
For each deficient section, engineering standards are applied to identify potential improvements that would correct the deficiencies. Pavement condition and V/C ratio are the trigger deficiencies that will cause HERS-ST to consider improving a section. Once a section is considered, HERS-ST will evaluate improvements that correct other deficiencies that may be present.
HERS-ST will identify up to six improvement "types" for correcting all or a portion of a section's deficiencies. These improvement types involve pavement, widening, and/or alignment corrections and can be done in combination with each other. HERS-ST therefore has numerous combinations of improvement types from which to select.
The HERS-ST logic relies on the application of benefit-cost analysis to evaluate and select the best set of section improvements for system-wide implementation. Benefit-cost analysis involves the comparison of expected benefits (the reduction in user, agency, and societal costs), over the life of the improvement, with the cost of implementing the project. Improvements will influence the costs and benefits accruing directly to highway users because of changes in travel-time, crash, and vehicle operating costs. Improvements also will impact State highway agency costs associated with maintenance. Also, highway improvements may impact non-highway users by, for example, reducing motor vehicle emissions as a result of lower traffic congestion.
HERS-ST addresses two questions when evaluating improvements for possible implementation: 1) should a section be improved in the current FP, and 2) if a section is to be improved in the current FP, which option from the set of candidate improvements would be the best to implement? HERS-ST selects improvements on the basis of the ratio of the net present value of each improvement option's incremental benefits to the present value of its incremental costs. Potential improvement options are sequentially compared until the optimal alternative is identified.
The benefit-cost result for each tentatively selected project is used to rank, on a system-wide basis, the best improvements selected for each section. If benefits exceed costs, and funds are available, the option will be selected. For a benefit-cost run where the BCR threshold is set to 1.0, the best improvement for each section is implemented first. When constrained by budget or performance goals, HERS-ST ranks the best improvements for all sections by their BCRs, then implements the improvements one at a time, by BCR rank, until the constraint is met.
If you provide data in the optional State Improvements Data file, HERS-ST will use the improvements you specify in this data to override some or all of the improvement decisions recommended by the model for individual highway sections. This means that you may specify a particular type of highway improvement for implementation, when the improvement will be made, how much it will cost, and what impact the improvement will have on the capacity of the segment. When you operate the model in Override mode, it will select the user-specified improvements regardless of whether they are cost-effective as measured in the model's internal calculations. This feature allows HERS-ST to reflect the often more complete information available to State DOT officials, relative to that found in the HPMS database.
There are two types of HERS-ST outputs, Results and Reports. HERS-ST generates the results automatically, which are then used to generate user-selected reports. Both types of output provide information concerning the impact of highway capital improvements on system condition and performance and on highway users. In particular, the model is capable of reporting on benchmarks relating to performance measures, such as changes in user costs, as well as vehicle- miles-traveled under deficient conditions.
HERS-ST produces estimates of justifiable expenditures for each FP by functional class (e.g., interstate, arterials, collectors) and improvement type, subject to the parameters the model is given and any constraints that are imposed on the solution.
When HERS-ST processes an analysis, it places a copy of the settings used for that specific analysis and the outputs of that analysis in the Results area. In HERS-ST, this combination of settings and outputs is a Scenario. A HERSST scenario may contain one or more Runs of an analysis. A Scenario with at least one Run is always created. You can also perform Sensitivity and Continuation analyses, which will produce multiple sets of Runs within a Scenario. The HERS-ST analysis can generate the following types of results data.
System Conditions. The System Conditions contain summary data on the initial conditions of the system as well as the state of the system at the end of each funding period. Each data element is summarized by functional class in addition to rural, urban, and overall totals.
Improvement Statistics. The Improvement Statistics contain aggregate information of the implemented improvements for each funding period and for the entire analysis period. Each data element is summarized by functional class in addition to rural, urban, and overall totals.
Section Conditions. Section Conditions contain information describing the condition of each section at the end of the funding period along with information about all improvements that have been selected and the effect of these improvements.
As illustrated in Figure 2-1, you can create four different types of outputs using the results of the HERS-ST analysis:
Charts. The Chart feature allows you to create complex charts and graphs to compare information from one or more analysis runs. You can create many types and styles of charts using the extensive formatting features and save your chart designs to a template library for reuse.