HERS-ST Highway Economic Requirements System - State Version: Technical Report - Chapter 4: Design of Improvement Alternatives
The highway improvements analyzed by HERS consist of various combinations of pavement, widening and alignment improvements. The user has the option of specifying improvements for each section by using the "override" mode. Otherwise, HERS assesses each section for deficiencies, and then designs improvements to correct these deficiencies.
This chapter begins with a discussion of user-specified improvements and the override mode. The second part of this chapter discusses the improvements that HERS designs, including: the section characteristics and types of deficiency levels that HERS utilizes in determining whether potential improvements should be evaluated; a definition of the HERS improvement types and the procedures used in designing them; and the effects of the HERS improvements upon section characteristics. The chapter closes with discussions of two aspects of adding or widening lanes: the application of the widening feasibility parameters, and the use of the capacity model to determine how many lanes are needed in the design year.
In override runs, the HERS-ST user has the ability to override some (or all) of the decisions HERS makes regarding the selection of improvements, the initial cost of these improvements, and their effects on capacity. In particular, for any section, the user can specify:
- That a particular type of improvement is to be made in a particular FP;
- The initial cost of such an improvement;
- Its effect on capacity; or
- That improvements are to be made only in specified FPs.
Override runs make it possible to override HERS-ST decisions on the basis of specific knowledge of the feasibility of particular improvements selected by HERS-ST or knowledge of unusual costs (e.g., for replacing bridges) that would be incurred in implementing these projects. These runs also make it possible to require that several related improvements (such as widening a given highway) be scheduled for the same FP or in consecutive FPs (because HERS analyzes individual sections in isolation, it is not currently capable of recognizing the relationship between such improvements).
Override runs also make it possible for HERS-ST users to specify improvements not selected by HERS. These improvements can be either HERS-type improvements (pavement, widening, or alignment improvements) or other types of projects (such as intersection modification or grade separation). In the latter case, the user must specify both the initial cost of the project and its effect on capacity. For HERS-type improvements, the user has the choice of providing cost and/or capacity specifications or allowing HERS to estimate these quantities.
Section 4.1.1 describes how users can specify improvements that should or should not be made and, optionally, specify the initial costs of these improvements and/or their effects on capacity. Section 4.1.2 describes some of the output describing the effects of such user-specified improvements. Finally, Section 4.1.3 provides details about how HERS-ST estimates the costs and benefits of user-specified improvements as well as the incremental costs and incremental benefits of substituting a more aggressive improvement for a user-specified improvement.
For a given section, user-specified improvements are described in an optional "State Improvements," or "StateImp" file. The name of this file is entered on the File Specifications screen of the HERS GUI Control Model or as the STATEIN input in PPSPEC.DAT. The HERS preprocessor reads this file and writes a binary version that is passed to the main HERS program.1
Each record of the StateImp file describes one or more improvements for a single highway section in chronological order. This information can be entered using the State Improvements data window of the HERS GUI, or it can be entered directly into the StateImp file using a comma-delimited format. Each record of the file contains 6n+3 fields, where n is the number of improvements described. The contents of the first 10 fields of this file are shown in Table 4-1.
|Continue for up to 10 improvements|
|1. Number of improvements||Integer|
|2. County Code||Integer|
|3. Sample Identifier||Alphanumeric|
|4. Year of First Improvement (four digits)||Integer|
|5. Type of improvement||Integer|
|6. Override Flag||Integer|
|7. Cost of Improvement||Decimal|
|8. Lanes Added||Integer|
|9. Increase in Capacity||Integer|
|10. Year of Second Improvement||Integer|
1. The GUI handles the passing of the binary file automatically. If the GUI is not being used, enter the name of the file to be created as the STATEOUT input of PPSPEC.DAT and the STATEIMPS input of RUNSPEC.DAT.
The first field of each record specifies the number of improvements described. A maximum of 10 improvements can be described in any record.
The next two fields contain the county code and the Sample Identifier (from HPMS Field 47). These fields are used to match the StateImps record with the corresponding HPMS record describing the section in question.2
User-specified improvements are entered in chronological order in sets of six fields (Fields 4 - 9, 10 - 15, etc.).
For the first improvement, a particular type of improvement is specified for a particular FP by setting the year (Field 4) to any year in that FP and identifying the improvement type in Field 5. For HERS-type improvements, the improvement type is specified using the codes in Table 8-3.3 Other types of improvements (such as intersection modification or grade separation) should be assigned codes that are divisible by 20 when they are not combined with pavement, widening, or alignment improvements. The combination of a non-HERS-type improvement with pavement, widening, and/or alignment improvements should be assigned a code that is the sum of a code that is divisible by 20 and the appropriate code from Table 8-3.4 Improvements that are not HERS-type or are combinations of HERS-type and non-HERS-type are called special improvements by HERS-ST.
If the first improvement involves adding lanes, the number of lanes to be added is specified in Field 8. An entry in this field is required for improvement types that end in 4, 5, 8, or 9; and this field only has an effect for improvement types that end in 4, 5, 8, or 9 (i.e., the lanes specified in Field 8 are added only if an "add lanes" improvement is specified). On two-way roads, HERS-ST normally assumes that, after the addition of lanes, the number of travel lanes will be even, and the current system does not have procedures for estimating capacity if the number of travel lanes will be odd. Accordingly, if the resulting number of lanes will be odd, the increase in capacity must be specified by the user in the last of the fields describing the improvement (Field 9 for the first improvement). Otherwise, a message is printed and the number of added lanes is adjusted appropriately (if an odd number of added lanes is specified, the number is increased by one; otherwise, the number of added lanes is reduced by one). If lanes are to be added but the lanes-added field is zero, HERS-ST adds either one lane (on one-way facilities) or the minimum number of additional lanes that will produce an even number of lanes (on two-way facilities), and a message to this effect is printed.
The initial costs of a user-specified improvement may be provided (in thousands of dollars), and the change in peak-hour capacity may be provided (in passenger-car equivalents per hour). For the first user-specified improvement, Fields 7 and 9 are used. For rural two- and three-lane roads, capacity changes should be specified as changes in two-way capacity; for all other roads they should be specified as changes in peak-period, peak-direction capacity. For HERS-type improvements, if these fields are blank or zero, the HERS-ST estimates of improvement costs and/or new capacity will be used. For all special improvements, these fields must contain non-zero values.
2. The match will be unique if all HPMS records are from a single state. If data from multiple states are used in a single run, there is a small probability that the same County/Sample-ID pair will be used to identify sections in more than one state. In this case HERS-ST 2.0 will arbitrarily match the StateImps record to the first HPMS record that it finds.
3. For unpaved sections, all improvements should entail pavement reconstruction. HERS treats user-specified improvements that do not entail pavement reconstruction as entailing reconstruction and prints a warning.
4. For example, the user might use an improvement type of 20 (or 40) to represent construction of an overpass. The construction of an overpass combined with resurfacing the entire section would then be represented by Type 21 (or 41).
The override flag (Field 6 for the first user-specified improvement) is used to indicate whether HERS-ST has any leeway in modifying a user-specified improvement. For HERS-type improvements (Types < 20) and pure non-HERS-type improvements (Types divisible by 20), this flag may be set to either zero or one. For improvements that are combinations of HERS-type and non-HERS-type improvements (Type greater than 20 and not divisible by 20), this flag must be set to one.
If the override flag is set to one, the project is selected just as described in the set of six fields. If it is set to zero, these fields describe the minimum improvement that will be selected. In this case, HERS-ST may identify a "more aggressive" improvement that warrants evaluation; i.e., an improvement that incorporates more widening than requested and/or also improves the section's alignment. If any more aggressive improvements are identified, the incremental benefits and costs of these improvements are estimated and used for determining whether any of the additional options should be implemented.
In addition to controlling whether HERS-ST can modify a user-specified improvement, the override flag controls the insertion by HERS-ST of improvements prior to the implementation of a user-specified improvement. If the override flag for the next user-specified improvement is one, HERS-ST does not consider any improvements until after the corresponding FP. On the other hand, if it is zero, HERS-ST considers the possibility that a pavement or widening improvement may be warranted in an earlier FP and, if so, an appropriate improvement is selected for the earlier period. In this case, if the user-specified improvement is of Type 1, 2, or 6 (i.e., the kind of improvement that can be analyzed by a Pavement Management System), the two improvements are combined and assigned to the earlier FP (but only if the user-specified improvement would otherwise be implemented prior to the end of the last FP to be analyzed during the run). When two improvements are combined in this way, any subsequent user-specified improvements with override flags of zero are advanced appropriately. If the user-specified improvement is not of Type 1, 2, or 6, its timing is not adjusted, but HERS-ST prints a message to alert the user to the possibility that the number of improvements selected for the section may be more than are warranted.
As implied in the preceding paragraph, the override flag can be used to prevent HERS-ST from selecting improvements for a section in one or more FPs. For this purpose, a set of six user-specified improvement fields is used with an appropriate year in the year field, an improvement code of zero, and an override flag of one. If these fields represent the first user-specified "improvement" to be implemented (i.e., if they are Fields 4 - 9), no improvements will be considered by HERS-ST until after the FP corresponding to the coded year. If there are one or more earlier user-specified improvements coded, the period when no improvements are considered starts after the last of the user-specified improvements. For example, if the first user-specified improvement is requested for FP 1, the second set of improvement fields can be used in this way to specify that no subsequent improvements be considered until FP 4 (by setting the year in the second set of improvement fields to a year that falls in FP 3). Similarly, if the first set of improvement fields contains an improvement code of zero, an override flag of one, and the same year (in FP 3) in the year field, no improvement will be considered until FP 4.
In each FP, HERS-ST automatically selects all user-specified improvements for implementation regardless of their benefit/cost ratios (BCR). If a funding constraint is in effect, the cost of these improvements is subtracted from the budget for the FP before any consideration is given to selecting other improvements or choosing to go beyond the user-specified minimum improvement for any section for which such an improvement is specified. If the cost of implementing all user-specified improvements exceeds the funding constraint for the FP, a message is printed and the funding constraint for the next FP is reduced accordingly. Similarly, if a performance goal is in effect, the benefits of user-specified improvements are taken into account before considering any improvements or improvement options that are not user specified.
Descriptions and evaluations of all improvements selected for a particular FP are contained in the section file (SECNSnn.OUT) produced for that FP. The contents of this file are described in section 8.3 "Section Output Files" on page 8-11. For any section, comparisons of the benefits and costs of alternative improvements can be obtained by specifying each alternative in a separate override run and comparing the resulting evaluations in the improvement files that are produced.
This section provides details of how HERS-ST estimates the user benefits and improvement costs of user-specified improvements and also the corresponding incremental benefits and incremental costs of replacing a user-specified improvement by a more aggressive improvement.
If the cost of any user-specified improvement is specified, HERS-ST uses that cost as the cost of the improvement. Otherwise, if the improvement is a HERS-type improvement, HERS-ST estimates the cost using the HERS procedure for estimating improvement costs (as described in Chapter 6, "Capital Cost of Improvements."). If no cost is provided for a special improvement, HERS-ST prints a warning message and sets the improvement cost to a default value.5
When evaluating the possibility of replacing a HERS-type user-specified improvement (e.g., resurfacing, as identified by a State's Pavement Management System) by a more aggressive improvement (e.g., resurface and add lanes), HERS estimates the incremental cost of replacing the former improvement by the latter one. This incremental cost is estimated as the difference between estimates of the costs of the two improvements that are both obtained using HERS' procedure for estimating improvement costs (regardless of whether the user has provided an exogenous cost estimate for the user-specified improvement).
Consider the possibility of replacing a non-HERS-type user-specified improvement (Type divisible by 20) by a combination of that improvement and a HERS-type of improvement. The cost of the combined improvement is estimated by using the HERS procedure for estimating improvement costs to estimate the cost of the HERStype of improvement, and adding this cost to the cost of the original user-specified improvement. Thus, the resulting estimate of the incremental cost of adding the HERS-type improvement ignores any efficiencies obtained by implementing both improvements simultaneously; and so there may be a tendency to overestimate the incremental cost. If the first improvement applies only to an intersection or interchange and the second applies to an entire section, this effect is likely to be fairly small6 and so may be ignored. If both improvements apply to the entire section, however, the effect may be more significant. For this reason, HERS-ST will tend to overestimate the incremental cost of adding a HERS-type improvement to a user-specified improvement that affects an entire section. Also for this reason, HERS-ST requires that the override flag be set to one for user-specified improvements that combine HERS-type and non-HERS-type improvements.
HERS-ST estimates the user benefits of an improvement as the net reduction in user costs resulting from changes in the physical characteristics of the improved section (including increases in the number of lanes) and from the resulting increases in capacity and average speed.
In the case of user-specified improvements, any increase in the number of lanes must be specified by the user and any increase in capacity may be specified. The user benefits of a non-HERS-type improvement (Type divisible by 20) are estimated entirely from these two increases. For such improvements, if both fields are zero, estimated benefits will be zero (if only the second field is zero, however, increased capacity will be estimated from the increase in the number of lanes.)
user-specified improvements that either are purely HERS-type (Type < 20) or are a combination of HERS-type and non-HERS-type (Type not divisible by 20), may produce other changes in the physical characteristics of the section. These are simulated by HERS-ST, and thus they provide another potential source of information for estimating user benefits. For these sections, the estimates of user benefits reflect any non-capacity effects of these changes plus either the user-coded change in capacity or, if the capacity change is not coded, the HERS-ST estimate of change in capacity.
When evaluating the possibility of replacing a user-specified improvement by a more aggressive improvement, HERS-ST estimates the incremental user benefits of the replacement by analyzing the effects of the replacement on the physical characteristics of the section and the resulting effects on user benefits. This process is straightforward when the user does not specify the capacity effects of the user-specified improvement, but it requires some clarification for the case in which the user does specify these effects. In the latter case, we distinguish several different estimates of capacity:
- Estimated capacity before implementation of the user-specified improvement;
- Estimated capacity after implementation of this improvement - obtained by adding the user-coded capacity effect of the improvement to (1);
- A separate estimate of capacity after implementation of the improvement that is produced by HERS-ST entirely from the physical characteristics of the section after improvement (ignoring the user-coded capacity effect); and
- HERS-ST's estimate of capacity after implementation of the more aggressive improvement (obtained entirely from the physical characteristics of the section without reference to the user-specified capacity effect of the original improvement).
6. One potential efficiency is a reduction in the disruptive effects of highway construction. However, because the costs of such disruption are not currently estimated by either HERS or HERS-ST, there is no need to adjust for the reduction in disruption.
The benefits of the replacement are then estimated by using (2) as estimated capacity without the replacement, and (2) + (4) − (c) as estimated capacity with the replacement.7
HERS will design improvements for any deficient section for which the user has not set the override flag in the state improvements file to one. HERS improvements are designed by the model in response to deficient section conditions. Two deficiencies - capacity and surface condition (PSR) - are considered to be "triggering" deficiencies: HERS will not consider improving a section unless at least one of these two conditions is deficient. HERS recognizes other potentially deficient section characteristics (such as lane width, shoulder type, and alignment), but will not improve a section based only on these less urgent shortcomings.
This section discusses criteria HERS uses to identify deficient sections; the types of HERS improvements and the process of designing them; and the effects of implementing these improvements on section characteristics.
HERS recognizes a section's need for improvement by comparing its characteristics to user-specified deficiency levels. HERS distinguishes up to three degrees of deficiency that might exist for eight characteristics of each highway section. The eight characteristics are:
- Pavement condition;
- Surface type;
- Volume/Capacity (V/C) ratio;
- Lane width;
- Right shoulder width;
- Shoulder type;
- Horizontal alignment; and
- Vertical alignment.
The three degrees of deficiency are identified by three user-specified levels: DL (deficiency level); SDL (serious deficiency level); and UL (unacceptability level). The roles of these three levels in the HERS improvement-selection procedure are:
- If the DL for a particular characteristic of a section is violated, HERS will analyze the benefits and costs of potential improvements that would correct this condition. If the resulting benefit/cost ratio of such an improvement is high enough, it may be selected to be implemented.
- If the SDL for a particular characteristic of a section is violated, then only improvements that correct this condition are evaluated by the HERS benefit-cost analysis procedure.
- If the user has specified that improvements are mandatory if the UL for a particular characteristic is violated, then an improvement that corrects this condition is normally selected automatically (see section 188.8.131.52, "Addressing Unacceptable Conditions: the Optional First Pass."). The B/C ratio for the improvement is considered by HERS only if the limiting constraint (whether funds available or system performance level) is insufficient to correct all such conditions.
The values used by HERS are contained in an external ASCII file (DLTBLS.DAT) for convenient user access and modification. The user may modify the DLs through the Parameter Model screens of the HERS-ST GUI. The user can access the SDL and UL settings using the GUI's "advanced mode."
Suggested default values for the ULs, SDLs, and DLs for the eight section characteristics are presented in Appendix A (these values were used in processing for the 2002 C&P Report). As indicated in the exhibits, for rural sections HERS allows separate values to be specified for three terrain types and eight combinations of functional system and average daily traffic (AADT). For urban sections, HERS allows separate values for each of five functional systems.
In the case of pavement condition (Table A-1, "Default Pavement Condition Criteria (PSR)"), SDLs are not shown, but a set of "reconstruction levels" (RLs) are. SDLs for pavement condition are not needed because all improvements involve either resurfacing or reconstruction, and, in HERS, only one of these two improvement options are considered for a section in any funding period. The pavement option considered is resurfacing unless PSR is below the RL or certain surface-type deficiencies (specified in Figure 4-2) exist.8
The tables in Appendix A also show user-specified Thresholds and, in several of the tables, the design standards (DS) for rural sections used by HERS. The design standards used for median width and for curves and grades are shown in Table A-10, "Default Design Standards For Median Width (Feet)," and Table A-11, "Default Design Standards For Curves and Grades." The design standards used for urban sections are shown in Table A-12, "Default Design Standards for Urban Sections."
HERS does not use the USTs in selecting improvements; however the shoulder type USTs are used as design standards when shoulders are improved and the lane width USTs are used to specify the lane width following reconstruction of an unpaved section. HERS uses the USTs to develop some summary statistics.
8. For medium and high-type pavement, PSR is the sole determinant of whether a section should be reconstructed; and for low-type pavement, it is the primary determinant. HERS is unable to take into account other influences on the reconstruction decision (e.g., height of the pavement crown), because they are not currently described in the HPMS database.
The three paragraphs below provide some general discussion of the default values for the unacceptability levels, deficiency levels, and serious deficiency levels, respectively, and the effects of using values that are more or less stringent.
The DLs identify deficiencies that warrant analysis by HERS. Logically, the DLs may be set at any value between the SDLs and the design standards. Relatively relaxed DLs (i.e., specifying relatively low levels of service) will limit the number of potential improvements analyzed by HERS and decrease computation time; while more stringent DLs will require HERS to analyze a larger number of potential improvements and may permit HERS to find a more cost-effective set of improvements to be implemented. HERS users should be aware that the optimal set of DLs will actually vary with the particular objective function used (that is, the type of analysis requested) and with the size of the highway-improvement budget (the optimal DL settings will get more stringent as the budget increases). The DLs presented in Tables A-1 through A-9 are the values used in the 2002 C&P Report.
The SDLs are used by HERS to limit the number of alternative improvements analyzed for a given section. Logically, the SDLs may be given values between the UL values and the DL values. If all SDLs are set equal to the corresponding DLs, no more than one improvement will be analyzed for each section in a given funding period, and any improvement analyzed will address all deficiencies identified for the section. If all SDLs are set equal to the corresponding ULs, up to six different improvements may be analyzed for each section (these consist of a pavement option with or without the improve alignment option and with zero, one or two widening options). The settings used for the SDLs thus will have a significant effect on the computation time required by HERS.
The SDLs have another potentially significant effect. When an SDL is violated for a particular section for which no UL is violated, any improvement evaluated for the section must correct the specified serious deficiency. This restriction will increase the cost of improving the section, but it also decreases the likelihood that the section will be improved without correcting all serious deficiencies (it does not guarantee that all serious deficiencies will be corrected since, if mandatory improvements are enabled and an unacceptable condition develops, HERS may correct the unacceptable condition without correcting other serious deficiencies).
This second effect of the SDLs suggests that it may be appropriate to set all SDLs to the represent minimum levels of service. Many of the suggested SDL default values presented in Tables A-1 through A-9 are, in fact, set to the HPMS' minimum tolerable conditions values.
A review of Table A-1, "Default Pavement Condition Criteria (PSR)," indicates that the default UL values for pavement condition are slightly below the reconstruction level, and a review of Tables A-3 through A-9 indicates that the other UL values represent conditions somewhat worse than the "minimum tolerable conditions" formerly defined by the HPMS. The UL values are called into play only when the user has specified that the model shall give priority to mandatory improvements. Mandatory improvements to improve sections with conditions below the ULs are not required to pass the minimum benefit-cost ratio threshold. Mandatory improvements are not used in preparation of the Conditions and Performance Report.
The highway improvements considered by HERS consist of resurfacing or pavement reconstruction, possibly combined with some type of widening and/or alignment improvement. Schematically, these improvement types can be viewed as being obtained by selecting one "improvement option" from each of the columns of Table 4-2. There are 32 possible combinations for the options in the three columns of the table, and eight possible combinations for the options in the first two columns. However, as HERS corrects shoulder deficiencies when reconstructing pavement, it makes no distinction between pavement reconstruction with or without shoulder improvements. The result is 28 different "types" of improvement, or, if the third column is ignored, seven different "kinds" of improvement.
|0. Resurface||0. None||0. No change|
|1. Reconstruct||1. Improve shoulders||1. Improve curves|
|2. Widen lanes||2. Improve grades|
|3. Add lanes||3. Improve curves and grades|
Table 4-2 shows three distinct alignment options: improve curves, improve grades, or improve both. In HERS, if curves (respectively, grades) are in "unacceptable" condition (as defined elsewhere in this chapter) but grades (respectively, curves) are not, then an improvement that improves curves (respectively, grades) to the design standard but does not modify grades (respectively, curves) may be selected. Otherwise, only alignment improvements that result in improving both curves and grades to the design standard are considered.
Each of the seven kinds of improvement are described briefly in Figure 4-1. Within each group, the improvements are listed in decreasing degree of aggressiveness.
HERS uses an additional set of extra-cost options to improve substandard urban freeways to design standards. The four options are: surface shoulders; improve access control to full; upgrade median type to positive barrier; and widen median to design standard. The appropriate improvements are only implemented on substandard urban freeways undergoing pavement reconstruction; sections being resurfaced are not upgraded in this manner.
The HERS model differentiates between lanes added at "Normal" and "High" cost. New lanes are added at normal cost when they do not violate the state-supplied Widening Feasibility code (WDFEAS) for the section. The user has the option of setting the Federal Override (WDFOVR) value to add lanes beyond those permitted by the state code up to the maximum lane limit (MAXLNS). These lanes are added at high cost. It is possible for a section to be improved by the addition of lanes at both cost levels: HERS reports these improvements as high cost lanes in the output statistics.
During each funding period, HERS is designed to make two passes over the entire set of sample sections to identify improvements that might warrant implementation. The first pass is optional and is used to identify improvements to be implemented based upon engineering criteria, regardless of their economic desirability. During the first pass, for paved sections with unacceptable present serviceability ratings for pavement condition and for unpaved sections with unacceptable surface type or lane width, HERS selects an appropriate inexpensive improvement. If sufficient funds are available, all such improvements are implemented without any B/C analysis. Otherwise B/C analysis is used to select the improvements to be implemented. Note: this optional first pass is not used in the analysis for the C&P Report.
- Reconstruction with More Lanes - Complete reconstruction with the addition of lanes to the existing section. Lanes added in excess of the state-coded widening feasibility code are added at high cost - otherwise, lanes are added at normal cost. Shoulder and drainage deficiencies are corrected.
- Reconstruction to Wider Lanes - Complete reconstruction with wider lanes than the existing section. No additional lanes are added. Shoulder and drainage deficiencies are corrected.
- Pavement Reconstruction - Complete reconstruction without adding or widening lanes. Shoulder width increased to design standard if feasible, and any other shoulder or drainage deficiencies are corrected.
- Major Widening - The addition of lanes to an existing facility. Lanes added in excess of the state-coded widening feasibility code are added at high cost - otherwise, lanes are added at normal cost. This improvement includes resurfacing the existing lanes and other minor work such as shoulder and drainage work.
- Minor Widening - This improvement is similar to major widening except that the added width yields wider lanes or shoulders, but no additional lanes.
- Resurfacing with Shoulder Improvements - The overlay of existing pavement plus the widening of shoulders to design standards if feasible or the complete reconstruction of shoulders to provide additional strength. A minor amount of additional right-of-way may be acquired.
- Resurfacing - The overlay of existing pavement including bringing the shoulders up to grade including minor drainage work.
Figure 4-1. Kinds of Improvement
During the second pass, HERS identifies additional deficiencies as well as appropriate improvements to address these deficiencies. B/C analysis is then used to determine which of these improvements to implement.
The procedures used to identify improvements during each of these passes are presented below. As the procedure used during the second pass is central to the use of B/C analysis by HERS, that procedure is described first.
During the second pass, HERS identifies deficiencies on the basis of the user-specified DLs and SDLs. HERS identifies an improvement type only when a pavement or capacity deficiency exists in the current funding period. When such a deficiency exists, HERS generally will identify at least one improvement type that will address it (the exception to this rule is when the only deficiency is a capacity deficiency, and additional lanes are either not needed or cannot be added, and it is not possible to widen the section to correct any substandard shoulder or lane widths).
The procedure for identifying improvement types to address ordinary deficiencies consists of two components:
- Identification of one (or, in some cases, two) "aggressive improvement type(s)" that will correct all identified deficiencies; and
- Identification of any less aggressive improvement types warranting B/C analysis as possible alternatives to the first, more aggressive, improvement(s). These alternatives would address some but not all of the segment's deficiencies.
These two components of the procedure are discussed below.
The procedures for selecting pavement, width and alignment options are presented in Figure 4-2, Figure 4-3, and Figure 4-4 respectively. These three procedures, taken together, identify a set of options that define a maximum of two improvement types.
- Reconstruct if:
- PSR at the beginning of the funding period is less than reconstruction PSR;
- surface type is low and deficient, and a widening option is identified; or
- surface type is unpaved and:
- surface type is deficient; or
- a widening option is identified.
- Otherwise resurface if:
- PSR at the end of the funding period is deficient; or
- a widening option is identified.
Figure 4-2. Identification of Aggressive Pavement Option
When considering pavement options, HERS decides whether resurfacing or reconstruction is appropriate based upon PSR at the beginning of the funding period analyzed. This reconstruction level is set by the user. As shown in Figure 4-2, the consideration of reconstruction can also be triggered by the inadequacy of the current surface type.
When considering widening options, HERS may select both an "add lanes" option and, if appropriate, the "widen lanes" option as well. This is the only situation in which HERS will identify two aggressive improvement types for further analysis. This can occur only when:
- Additional lanes can be added;
- More lanes are needed in the design year, but not now;
- Widening lanes will increase capacity without correcting the design year capacity deficiency; and
- Reconstruction is needed.
HERS identifies the widen lanes option whenever the PSR is deficient, widening is feasible, and either lanes are needed but cannot be added or lane width is deficient. The HERS decision to widen lanes does not depend upon factors such as whether reconstruction is required, rural or urban location and whether or not the section is an urban freeway/expressway up to design standards.
- For unpaved sections:
- Widen lanes if a deficiency exists in lane width or (for collectors in the lowest volume category) in sum of lane width and shoulder width.
- For paved sections:
- Add lanes if lanes can be added and
- more lanes are neededa
- pavement requires reconstruction, and more lanes will be needed in the design year
- Widen lanes if lanes can be widened and
- more lanes are not needed, or lanes cannot be added and
- lane width is deficient, and PSR is deficient; or
- design hour V/C is deficient, and capacity would be increased by widening lanesb, and lane width is less than design standard; or
- If PSR is deficient, and design hour V/C is not deficient but will be in the design year, and capacity would be increased by widening lanesb, and lane width is less than the design standard, and
- section is an urban freeway by design type; or
- the reconstruction option has been identified solely as a result of a pavement or surface type deficiency.
- more lanes are not needed, or lanes cannot be added and
- Otherwise, improve shoulders if:
- shoulder width is less than the design standard, widening is feasible, design hour V/C is deficient, and capacity would be increased by improving shouldersb;
- shoulder width is deficient, widening is feasible, and PSR is deficient; or
- shoulder type is deficient and PSR is deficient.
- Add lanes if lanes can be added and
Figure 4-3. Identification of Aggressive Widening Options
- "More lanes are needed" means V/C is deficient both now and in the design year.
- On some sections with initial capacity coded in the HPMS data base, widening lanes or shoulders may not result in any increase in capacity (because the calculated capacity after widening is no higher than the coded capacity). For such sections, the "widen lanes" and "improve shoulders" options do not produce any benefits recognized by HERS. For such sections, these improvements are analyzed by HERS only if they address a "serious deficiency".
When considering alignment options, HERS considers both horizontal and vertical alignment improvement for rural sections and horizontal alignment improvements for urban principal arterials. This analysis is done only for segments for which complete information about curves and/or grades by class is available (the HPMS Field Manual requires this information for rural principal and minor arterials and urban principal arterials).
- For rural sections:
- Improve curves and/or grades if:
- curves and/or grades by class are specified,
- any pavement or widening option is identified, and
- horizontal or vertical alignment is deficient.
- Improve curves and/or grades if:
- For urban sections:
- Improve curves if:
- curves by class are specified,
- reconstruction is identified, and
- horizontal alignment is deficient.
- Improve curves if:
Figure 4-4. Identification of Aggressive Alignment Options
After identifying one (or two) aggressive improvement types that will address all of a section's deficiencies, HERS then identifies less aggressive improvement alternatives, and uses benefit-cost analysis to choose among all the alternatives. In general, less aggressive improvements can be derived directly from the most aggressive improvement by:
- Replace a widening option with no widening;
- Replace a widening option with a less aggressive widening option; or
- Replace improved alignment with no change in alignment.
Replacement rules 1 and 3 involve replacing a particular option with the corresponding "zero-level" option (as defined and numbered in Table 4-2, "Improvement Options"). If HERS had selected a single aggressive improvement type including both widening and an alignment improvement, these two replacement rules would identify up to three less aggressive alternative improvements for analysis (zero-option for widening, zero-option for alignment, and zero-option for both). If HERS had selected two aggressive improvement types for further analysis, these rules would identify up to four less aggressive alternatives, so up to six alternatives might be analyzed for some sections.
Although an exhaustive evaluation of all such alternatives may, at times, be of interest, it is likely that HERS users frequently will prefer that some of these evaluations be skipped in order to shorten run times. This is accomplished using the SDLs introduced in section 4.2.1 "Deficiency Criteria" on page 4-7. When the user-specified SDL for a section characteristic is violated, any improvement option designed to address the deficiency is treated as "required" and HERS will not analyze the zero-level alternative. If the SDL is not violated, any improvement designed to address the deficiency is treated as "non-required" or "optional," and the zero-level alternative to the option is analyzed (there is no SDL for pavement condition, since the decision on whether to reconstruct or resurface a section is primarily based on the reconstruction level. HERS does not directly compare reconstruction alternatives with resurfacing alternatives for an individual section).
Replacement rule 2 is used only in the special case in which a lower-level widening option (e.g., improve shoulders, or widen lanes) is required (because the SDL for shoulder-width or shoulder-type is violated), but a non-required higher-level widening option (e.g., widen lanes or add lanes) is included in the aggressive improvement identified by HERS. In this case, HERS considers the required lower-level widening option as an alternative to the aggressive improvement, rather than evaluating the zero-level alternative.
If the user has elected to enable ULs (unacceptability levels), and if the capacity of a section is expected to violate the user-specified UL during the expected design life for a pavement improvement being considered, a capacity improvement option is treated as a required accompaniment to that pavement improvement, regardless of whether the capacity SDL is currently being violated. In this case, the zero-level alternative widening option would not be evaluated. This requirement enables HERS to avoid a situation in which capacity becomes unacceptable at a time when resurfacing (or reconstruction) is not normally performed.
If all SDLs are set equal to the corresponding DLs, then normally only the "most aggressive" improvements identified by the procedures of Figure 4-2 through Figure 4-4 are analyzed. If the SDLs are relaxed completely (that is, the SDL for the V/C ratio is set high and all other SDLs are set to zero), all alternatives generated by replacement rules 1 and 3 are evaluated. The implications of how the user chooses to set the DL, SDL, and UL levels are discussed more fully in section 184.108.40.206 "Deficiency Levels" on page 4-9.
The user may instruct HERS to identify unacceptable conditions and accord them a greater priority for correction than serious deficiencies. This option is available on the "Run Specifications" screen of the Control Model sequence, and is titled "Give priority to mandatory improvements."
In most instances, the improvements selected to correct the unacceptable conditions will be implemented without being subject to B/C analysis. When the analysis is constrained by available funds, a portion of the available funds may be designated for the correction of unacceptable conditions. In the case of paved sections, whenever a section is found to have an unacceptable PSR, an appropriate inexpensive improvement that addresses all unacceptable conditions on the section is identified. In the case of unpaved sections, whenever a section is found to have unacceptable surface type or lane width, an appropriate inexpensive improvement that addresses all such conditions is identified. The procedure for identifying options defining these improvements is presented in Figure 4-5.
Paved sections with unacceptable PSR and unpaved sections with unacceptable surface type or lane width will be referred to as sections with "triggering unacceptabilities". The procedure presented in Figure 4-5 is designed to identify all such sections and, except for the case in which the V/C ratio is unacceptable, the procedure is designed to identify the least expensive of the available HERS improvements that will correct all unacceptable conditions on such a section. In the case of an unacceptable V/C ratio, the procedure selects the most aggressive widening option warranted by the section's characteristics.
When a section has unacceptable pavement, the improvement identified by the procedure shown in Figure 4-5 will generally be selected to improve the section.9 However, the implementation of mandatory improvements is handled slightly differently depending on the user's analytical objective, as discussed below.
An important consideration when evaluating an "Add Lanes" improvement is the number of lanes being added, as this both determines the cost of the improvement and is the prime factor governing the user cost benefits of the improvement. The HERS version 4.n model may consider either one or as many as five different possibilities when adding lanes to a section. This latter option is called the "incremental number of lanes" option. The model determines the number of possible lane options in two steps: first, it determines the number of lanes required in the design year; and second, it identifies as many as four additional lane options. HERS then designs potential add lane improvements for each of the identified options for evaluation via cost-benefit analysis.
9. It should be noted that (as currently implemented), for paved sections, only pavement-related conditions trigger the correction of unacceptable conditions; but, when unacceptable pavement conditions are corrected, all other unacceptable conditions are corrected as well. This procedure, in conjunction with the procedure for addressing serious deficiencies, guarantees that any non-pavement-related unacceptable conditions will be corrected whenever the pavement of a section is improved. However, except when warranted by benefit-cost analysis, these conditions will normally not be corrected as long as the section's pavement remains in reasonably good condition.
- For paved surfaces:
- If at the end of the funding period the PSR is unacceptable:
- Reconstruct if surface type is low and unacceptable;
- Otherwise, reconstruct if at the beginning of the funding period the PSR is below the reconstruction PSR;
- Otherwise, resurface.
- If resurfacing or reconstruction has been selected, then:
- For rural sections, improve curves and/or grades if horizontal and/or vertical alignments are unacceptable, and curves and/or grades are specified by class;
- For urban principal arterial, improve curves if horizontal alignment is unacceptable, and curves are specified by class;
- If V/C is unacceptable:
- Add lanes if more lanes are needed and can be added;
- Widen lanes if lanes can be widened:
- if lane width is unacceptable; or
- if lane width is less than the design standard, and widening lanes will increase capacity, and widening shoulders will not make V/C acceptable;
- Otherwise, improve shoulders if shoulder width is less than design standard and shoulders can be widened and widening shoulders will increase capacity.
- Widen lanes if lane width is unacceptable and lanes can be widened;
- Improve shoulders if shoulder width is unacceptable and widening is feasible;
- Improve shoulders if shoulder type is unacceptable.
- If at the end of the funding period the PSR is unacceptable:
- For unpaved surface types:
- Widen lanes and reconstruct if lane width is unacceptable and lanes can be widened;
- Reconstruct if surface type is unacceptable.
Figure 4-5. Identification of Improvement Options for Addressing Unacceptable Conditions
When estimating the future traffic volume for the purpose of determining lane requirements, HERS does not apply demand elasticity.10 Instead it projects the geometric rate of growth AADTGR (see Equation 5.55 on p. 5-53) as in Equation 5.56 on p. 5-54, using the "fully elasticized" AADT at the beginning of the current funding period for VADJ, and the number of years from the time of that AADT to the "design year" as the "length of the funding period" (LFP).11 The design year is the length of the design period (specified by the user in the parameter file; the default value is 20 years) from the point of implementing the improvement, which is at the middle of the current funding period.
The design number of lanes normally is the minimum number necessary to achieve a design year volume/capacity ratio that satisfies the user-specified widening standard.12 The capacity used in this ratio is the peak capacity that would be produced by adding the specified number of lanes. The design number of additional lanes (DNADDL) is the total number of lanes in the design year minus existing lanes, and may be reduced as a result of limitations on widening feasibility or on the maximum numbers of lanes allowed. This number is user-specified for each functional class and is limited to 99 (the value used for the 2002 C&P Report). Also, HERS limits sections with stop signs to a maximum of two through lanes in each direction.
If the user has disabled the incremental number of lanes option, HERS only evaluates adding the design number of additional lanes.
When the user has enabled the incremental number of lanes option, the five options HERS may identify for a two-way road (where HERS adds lanes in pairs) are:
- Two lanes (when the section's widening feasibility code allows the addition of exactly two normal cost lanes and the design number of additional lanes (DNADDL) is not equal to two);
- Half of DNADDL;
- Three quarters of DNADDL;
- DNADDL; and
- DNADDL plus two (depending on capacity criteria).
For one-way roads, where HERS adds lanes singly, the options become:
10. In one sense, this mimics the non-elastic calculations of highway engineers. Practically, it avoids the computationally intense attempt to solve for elasticity when the future capacity is unknown.
11. The default value for the design period is 20 years, which is presumed to begin at the time the improvement is implemented. Thus, for the initial five-year funding period, the improvement is implemented at year 2.5, the design year is year 22.5, and the exponent used is 22.5.
- One or two lanes (the maximum number of normal cost lanes permitted by the section's widening feasibility providing that number is less than DNADDL);
- Half of DNADDL;
- Three quarters of DNADDL;
- DNADDL; and
- DNADDL plus one (depending on capacity criteria).
HERS doesn't add fractional lanes, rounds the numbers in options two and three, and doesn't consider duplicate cases. In many cases HERS will determine that only one option is suitable for benefit-cost analysis.
Option one is designed to prompt HERS to evaluate the lower cost option of adding only lanes at normal cost, and comparing this with options where some lanes are added at high cost. For two-way sections, this means a widening feasibility (WDFEAS) of four; for one-way sections, WDFEAS may be either three or four. This option becomes moot when the widening feasibility override (for the functional class) is set at less than five - in that instance, DNADDL would be limited to the number of normal cost lanes permitted by the section's widening feasibility code.
For HERS to consider options two and three, the options must identify numbers of additional lanes which are distinct from DNADDL and option one (if option one is identified). They must also be distinct from each other.
Option four is the design number of additional lanes. It is always identified as a potential add lanes improvement.
Option five compares three predicted capacities: the capacity needed to satisfy the forecast traffic on the section (NEEDCAP); the capacity predicted after adding DNADDL lanes (NEWCAP); and the capacity predicted after adding DNADDL less one or two lanes (TEMPCAP, for one-way and two-way roads, respectively). As an artifact of the procedure which determines DNADDL, NEWCAP will be greater than NEEDCAP, which in turn is greater than TEMPCAP. If the difference between NEWCAP and NEEDCAP is less than the difference between NEEDCAP and TEMPCAP, HERS will consider option five.
Table 4-3 illustrates how HERS applies the above-described logic to the problem of designing add lane improvements. Each block describes the parameters for a section and the number of lanes added for each active option. Inactive options (because they generate duplicate lane values or their governing conditions are not met) are not included in the block. All the examples are two-way highways, and as a simplification, all the lanes in the examples have a capacity of 1000. The widening feasibility override is set to five, allowing HERS to add lanes beyond those permitted by each section's WDFEAS code, but at high cost. NEWCAP represents the capacity of the existing lanes plus the minimum number of lanes needed to meet design year capacity requirement (in NEEDCAP). That minimum number of lanes is shown as option four. (Thus the existing number of lanes equals NEEDCAP divided by 1000 minus the lanes added under option four.) TEMPCAP is the capacity after adding two fewer lanes than option four.
The first block is a full-blown example: all the add lane options are active. Option four indicates that adding eight lanes would satisfy the NEEDCAP requirement. Because WDFEAS is four, two lanes can be added at normal cost, so option one will be considered. Options two and three duplicate none of the other options, and will also be evaluated. And since NEEDCAP is closer to NEWCAP than TEMPCAP, HERS will also consider option five. Note that each option adds two lanes at normal cost, with additional lanes added at higher cost.
The second block illustrates a section which differs from the first in only one circumstance: its widening feasibility is five, meaning that 3 or more lanes can be added at normal cost. Option one is not considered; HERS only designs and evaluates improvements adding four, six, eight and ten lanes. All the lanes are added at normal cost.
The third block shows the same section with a widening feasibility of one which, in concert with the override code of five, permits HERS to only add lanes at high cost. HERS again designs and evaluates improvements adding four, six, eight and ten lanes, but this time at high cost.
|WDFEAS||NEEDCAP||NEWCAP||TEMPCAP||Qualifying Option||Normal Cost Lanes Added||High Cost Lanes Added||Total Lanes Added|
The fourth block represents a four lane section where the additional lanes needed for the design year capacity are four (option four). Because WDFEAS is 4, two lanes can be added at normal cost, so HERS activates option one. HERS discards options two and three since, after rounding, they would duplicate options one and four. Option five is also activated due to the relative values of the three capacities. So for this section, HERS will design potential improvements that add two, four, or six lanes.
The fifth block presents a section similar to the fourth block, except that WDFEAS is five (allowing three or more lanes to be added at normal cost), and the needed capacity is less. Because WDFEAS is no longer four, HERS discards option one. Option two, at half the design year number of additional lanes, is no longer a duplicate of option one, so HERS activates it as two normal cost lanes. The conditions for option five do not hold, so HERS designs two potential add lanes improvements for this section, adding two or four lanes.
The effects of each HERS improvement are simulated by changing the description of the characteristics of the sample section. When evaluating potential improvements, HERS builds a temporary description of the section for each of the candidate improvements (when analysis extends for more than a single funding period, HERS builds a series of descriptions extending to the end of the benefit-cost analysis period). If HERS implements an improvement at the end of processing for a funding period, the altered description is saved for use in the following funding period.
HERS analyzes the effects of all improvements as if they are implemented instantaneously at the middle of a funding period instead of being spread throughout the funding period. Accordingly, by the end of a funding period, the PSR of a reconstructed section shows the effect of one-half period of pavement deterioration. The disruptive effects of improvements are not analyzed. Sections which do not undergo improvement during a funding period also have their section descriptions updated to reflect forecast changes in PSR and traffic volume. This "unimproved condition" is used as a base case in benefit-cost analysis.
Most of the effects of improvements on the "section data items" that form this description are shown in Table 4-4. The widening options in this exhibit are assumed to be accomplished without any change in rush-hour parking rules. Effects that occur only when lanes are added to rural sections are shown separately in Table 4-5 "Additional Effects of Adding Lanes on Data Items for Rural Sections.", The effects of alignment improvements on alignment-related data items (curves and grades, passing sight distance, and weighted design speed) and on pavement condition are presented in Table 4-6.
The additional effects of improving substandard conditions on urban freeways are presented in Table 4-7, "Effects of Addressing Substandard Conditions on Urban Freeways." In HERS, the changes shown in Table 4-7 are implemented, if feasible, whenever a substandard urban freeway undergoes pavement reconstruction.
The effects of improvements on widening feasibility are presented in Table 4-8, "Widening Feasibility Code Adjustments." Generally, when widening improvements are made, widening feasibility for future improvements is reduced. The exception is sections coded as having unlimited widening feasibility, for which widening feasibility is never reduced. The values shown assume that the override factor WDFOVR is set to 5, thus allowing widening in excess of the limits indicated by WDFEAS. However, the urban freeway upgrade and rural section upgrades are not affected by WDFOVR, and WDFEAS must have a value of at least 3 for an upgrade to be implemented. For these two upgrades, note that WDFEAS is checked after it has already been adjusted for the "main" improvement (see section 4.3 "The Widening Feasibility Model" on page 4-26 for additional discussion).
|Section Attributes||Improvement Type|
|Reconstruct With More Lanes (High or Normal Cost)||Reconstruct With Wider Lanes||Reconstruct Pavement||Major Widening (High or Normal Cost)||Minor Widening||Resurface W/Shoulder Improvement||Resurface Pavement|
|a. NC = No Change, and DS = set to Design Standard.|
|b. The design number of lanes is the number of lanes needed to accommodate projected traffic in the design year, and which would be added if widening were not constrained. The two constraints are widening feasibility and the maximum number of lanes allowed. This last number is user-specified and can vary with functional class. For the 1999 C&P Report, the maximum number of lanes for all functional classes was 99.|
|c. For an unpaved section the improvement reconstruct pavement results in paved lane widths equal to the user specified threshold.|
|d. Curbed sections remain curbed (with zero shoulder width) after improvements.|
|e. Shoulder only widened if feasible.|
|f. Changes in PSR specified in Table II-14 of HPMS/AP Technical Manual.|
|g. If low type pavement exists, resurfacing does not change the pavement type.|
|h. No change if recalculated capacity is lower than original capacity.|
|i. If the shoulders are widened, the value is recalculated.|
|Number of Lanesb||Design Number||NC||NC||Design Number||NC||NC||NC|
|Shoulder Typed||Existing or UST1||Existing or UST1||Existing or UST1||Existing or UST1||Existing or UST1||Existing or UST1||NC|
|Right Shoulder Widthd||DS||DS||DSe||DS||DS||DSe||NC|
|Pavement Condition (PSR Value)f||Recalculate||Recalculate||Recalculate||Recalculate||Recalculate||Recalculate||Recalculate|
|SN or D||NC or Increase||NC or Increase||NC or Increase||NC or Increase||NC or Increase||NC or Increase||NC or Increase|
|Median Width||Widen to design standard or to the extent feasible.|
|Median Type||Set to unprotected if median width is widened and median type is currently "none."|
|Access Control||If median is added access control is not full, set to partial.|
|a. The procedure used for determining the extent to which curves are lengthened is presented in Chapter 5.|
|b. HERS contains code for adjusting weighted design speed when there is no data on curves by class, but HERS does not consider horizontal alignment improvements when these data are not available.|
|c. The portion of the section with modified alignment equals the sum of the portions with substandard grades or curves (after lengthening) but is no greater than the length of the section.|
|Grades||Substandard grades are improved to design standard.|
|Curves||Substandard curves are lengthened and improved to design standard.a|
|Passing Sight Distance||(For rural two-lane highways only) improve to typical passing sight distance (from 1978 data).|
|Weighted Design Speed||Recalculate. If no data on curves by class, increase by 5 m.p.h.b|
|Pavement Condition||Obtain as a weighted average of the PSR on the portion of the section with modified alignmentc and the PSR indicated in Exhibit 3.3 for the remainder of the section.|
|a. Improvement to full control is assumed to require one lane of right-of-way.|
|b. Improvement of median width to design standard is assumed to require one lane of right-of-way.|
|Shoulder Type||Improve to surfaced.|
|Access Control||Improve to full control if feasible.a|
|Median Type||Improve to positive barrier.|
|Median Width||Improve to design standard if feasible.b|
This section discusses the HERS procedure for estimating pavement thickness resulting from resurfacing and reconstruction and for obtaining the corresponding structural number (SN) for flexible pavement. As discussed in section 5.1, "The Pavement Deterioration Model," SN is one of the influences on the deterioration rate of flexible pavement, and the deterioration rate of rigid pavement depends directly on pavement thickness. Additionally, the cost of alignment improvements is affected by pavement thickness.
In HERS, the design life of a pavement normally is taken to be twenty years. This value can be modified by the user; however, as pavement thickness is a function of the number of ESALS forecast during the design period, modifying this variable will affect pavement thickness (and hence pavement durability). Resurfacing or reconstruction cost will be affected only to the extent that some portion of the section has its alignment improved.
|Original WDFEAS Code||WDFEAS Code After Improvement|
|Resurface||Resurface with Improved Shoulders||Reconstruct||Widen Lanesa||Add Lanesb||Urban Freeway Upgradec||Rural Section Upgraded|
|where: NF = Not Feasible (the improvement is not made, and WDFEAS is not adjusted); NA = Not Applicable.|
|a. The adjustment is the same whether the section is resurfaced or reconstructed.|
|b. The adjustment is the same whether the section is resurfaced or reconstructed, and without regard for whether the lanes are added at normal or high cost.|
|c. Applies when correcting substandard conditions on urban freeways undergoing reconstruction. The model first tries to improve access control, and then to widen the median to the design standard. Neither improvement is implemented (and WDFEAS is not adjusted) if the condition is not substandard.|
|d. For rural sections when lanes are added, the model will widen the median and improve access control to "partial" if feasible. Unlike the urban upgrade, a single feasibility test (and adjustment) is made for both improvements.|
|e. If the shoulder is not curbed and is below the design standard, it is widened to and WDFEAS reduced.|
|f. Only one improvement is implemented. Access control is preferred - the median will only be widened if access control is already full.|
|g. If two lanes are added, the WDFEAS code is adjusted to 1 (no widening feasible). If one lane is added, the WDFEAS code is adjusted to 3 (one lane may be added).|
|h. Either, neither, or both of the two improvements may be needed or implemented.|
|i. This value will never be tested for the rural upgrade, as a WDFEAS of 4 would have been reduced as a result of the added lane(s).|
|1 (no widening)||1||1||1||1||1||NF||NF|
|2 (partial lane)||2||2 or 1e||2 or 1e||1||1||NF||NF|
|3 (one lane)||3||3 or 2e||3 or 2e||2||1||1f||1|
|4 (two lanes)||4||4||4||3||3 or 1g||4, 3, or 1h||NAi|
|5 (three or more lanes)||5||5||5||5||5||5||5|
The following subsections present the values of pavement thickness for reconstruction, simple resurfacing, and resurfacing and widening; and a final subsection presents the structural numbers used by HERS for reconstructed and resurfaced flexible pavements.
Assuming that the reconstructed pavement is designed and constructed as a new pavement structure, pavement thickness is a function of pavement material and traffic load. HERS assumes that reconstruction of either rigid or composite (flexible over rigid) pavement is performed with rigid pavement, and that reconstruction of flexible pavement uses flexible pavement. Thicknesses used by HERS for reconstruction of flexible (asphaltic concrete) pavements to a medium or high-type design standard are shown in Table 4-9 as are thicknesses for reconstruction of rigid (Portland cement concrete) pavements. For low-type flexible pavement, only a surface treatment is used.
|Forecast ESALs over Design Life||Pavement Type|
|Flexiblea b c||Rigidd|
|a. American Association of State Highway and Transportation Officials, AASHTO Guide for Design of Pavement Structures, Washington, D.C., 1986.|
|b. Thickness shown for flexible pavements are also used for resurfacing flexible pavements with a flexible overlay.|
|c. For low-type pavement, assume a surface treatment only.|
|d. E.J. Yoder and M.W. Witczak, Principles of Pavement Design, John Wiley, New York City, 1975.|
|50,001 - 150,000||2.5||6.5|
|150,001 - 500,000||3.0||6.5|
|500,001 - 2,000,000||4.0||8.0|
|2,000,001 - 7,000,000||5.0||9.5|
HERS assumes that resurfacing is always performed using a flexible overlay. For flexible overlays over flexible, composite, or rigid pavement, the overlay thickness used by HERS varies with traffic load in the same way as for reconstruction with flexible pavement. These thicknesses are shown in Table 4-9.
When resurfacing is combined with widening improvements, some part of the improved roadway will be built on land that is not already paved. In general, the newly paved area will be structurally compatible with the resurfaced roadway. HERS treats resurfacing with widening improvements as producing a single roadway whose characteristics are those of the original roadway after resurfacing.
HERS assumes that resurfacing or reconstruction never reduces the structural number (SN) of flexible pavement but may increase its value. To do this, a value of SN is obtained using an equation that approximates the relationship between SN and pavement thickness presented in Table IV-3 of the HPMS Field Manual13. This equation is:
|SN = NEWSNC1 + NEWSNC2 × Df|
|NEWSNC1||=||First NEWSNC entry from PARAMS.DAT,
default value is 1.5;
|NEWSNC2||=||Second NEWSNC entry from PARAMS.DAT,
default value is 0.75; and
|Df||=||Pavement thickness, in inches.|
If the resulting value is less than the original value of SN coded for the section, SN is set to that value.
When pavement undergoes reconstruction, HERS sets its PSR to a level determined by the section's location and surface type. These values, PSRREC, may be set by the user. Table 4-10 lists the default values.
For sections being resurfaced, HERS adds an increment to the PSR at the time of the improvement. This augmented PSR value is limited to maximum PSR values. These values, PSRINC and PSRRMX, may be adjusted by the user. The default values for incrementing PSR are shown in Table 4-11 and the default maximum PSR limits are shown in Table 4-12.
For any section, an improvement that combines pavement reconstruction with alignment improvements results in producing newly reconstructed pavement on the entire section. Such improvements produce a single type of pavement and a single PSR for the entire section.
On the other hand, improvements that combine resurfacing with alignment improvements produce a single PSR only in the (relatively rare) case in which the alignment of the entire section is improved. More commonly, such improvements produce one PSR for the portion of the section on which alignment does not change and a higher PSR on the portion that is reconstructed on a modified alignment. Furthermore, resurfacing of rigid or composite pavement is presumed to be performed with a flexible overlay (producing composite pavement), while the adjoining reconstructed pavement is presumed to be rigid. For both cases, HERS obtains a single combined PSR for the section by taking a weighted average of the PSRs on the two portions of the section, using the lengths of these portions of the section as weights. For the case in which part of the section receives a flexible overlay on composite or rigid pavement and part is reconstructed with rigid pavement, HERS uses the relative length of the two portions of the section to determine whether to treat the section as having rigid or composite pavement.
HERS considers an urban freeway substandard if (a) it is an Interstate or Other Freeways and Expressways (that is, functional class is 11 or 12), and (b) any one of the four deficiencies listed below applies:
- the shoulders are unsurfaced;
- access control is not full
- the median type is not positive barrier; or
- the median width is less than the design standard for urban freeways and expressways by design.
The default value for the urban median width design standard is set to 20 feet.
HERS will correct these deficiencies only on urban freeways which are being reconstructed. Improving access control to full and improving the median width to the design standard each require a lane of right-of-way as coded in the section's widening feasibility (WDFEAS) data item (this data item is part of the section's HPMS input record). If the availability of right-of-way is limited, precedence is given first to adding lanes, then to improving access control, and last to improving median width.
Six of the seven major HERS improvement options involve increasing the width of the roadway: adding lanes, widening lanes, and improving shoulders14. Additionally, widening the median and increasing access control are two upgrades which may be performed on rural sections (when lanes are added) and substandard urban freeways (when they undergo reconstruction) and which also increase the roadway width. "Widening feasibility" refers to the potential for increasing the total width of a particular section. HERS tracks the feasibility of widening each section, and updates the information whenever the section is improved. HERS uses the interaction of system and section-specific constraints to determine first, whether widening improvements can be implemented, and second, whether additional lanes will be added at "normal" or "high" cost (for information about improvements which widen the roadway, see Table 4-4 on page 4-21).
HERS also uses the widening feasibility model to determine whether sections undergoing alignment improvement are re-aligned at high or low cost.
There are four factors in HERS which limit the potential width of any section. First, the user specifies the maximum number of lanes (MAXLNS) allowed for each of the functional classes. The number may be as large as 99. It is applied only when determining the number of lanes to be added to a section; it is not used in determining the feasibility of widening existing lanes, widening shoulders or medians, or in improving access control. HERS does not remove lanes from existing highways in order to meet this limit.
Second, HERS will always build to an even number of lanes. Sections with an even number of existing lanes will receive additional lanes in even-numbered increments. Sections with an odd number of existing lanes will receive an odd number of lanes the first time HERS adds lanes to the section, and an even number of lanes should more lanes be added in a subsequent funding period.
Third, each section in the HPMS database includes a Widening Feasibility (WDFEAS) code indicating the extent to which the existing road may be widened. This state-supplied code reflects physical features along the section such as severe terrain, cemeteries and park land, and non-expendable buildings (large office buildings, shopping centers, etc.). It does not reflect restrictions due to current right-of-way, State widening practices, politics, or expendable buildings (single-family residences, barns, private garages, etc.). The widening feasibility codes are described in Table 4-13.
|1||No widening is feasible|
|2||Partial lane may be added|
|3||One lane may be added|
|4||Two lanes may be added|
|5||Three or more lanes may be added|
Fourth, the user specifies a system-wide Widening Feasibility Override (WDFOVR) code which corresponds to the widening feasibility codes in Table 4-13. In HERS-ST, the user may specify separate widening feasibility overrides for each functional system instead of being limited to a single override value for the entire system being analyzed (via the "Improvement Parameter" screen of Parameter Data). When the WDFOVR code is higher than a section's WDFEAS code, HERS may consider additional widening options which would ordinarily be precluded by the WDFEAS value. Lanes that are added up to the level specified by WDFEAS are treated as "normal cost" lanes. Additional lanes added based on the WDFOVR code are treated as "high cost" lanes, and are priced separately in the improvement cost file. High cost lanes are intended to represent extraordinary measures that could be taken to provide additional capacity such as double-decking a freeway, or constructing a new facility on a parallel route. Normal and high cost lanes are reported separately in HERS output.
The interplay of WDFEAS and WDFOVR is shown in Table 4-14. Each table entry lists the widening improvements HERS will consider for a section of the given widening feasibility code (WDFEAS, by column) for a specific value of the system variable WDFOVR (by row).
|WDFOVR||Widening Feasibility Code (WDFEAS)|
|where: SH = widen shoulders; WL = widen lanes; NCL = add normal cost lane(s); HCL = add high cost lane(s); Urb = on Urban freeways by design: improve access control to full and widen median to design standard; Rur = on Rural sections with added lanes: widen median and upgrade access control to partial.|
|a. When the existing facility has an odd number of lanes, add one normal cost lane.|
|b. When the existing facility has an odd number of lanes, add one high cost lane.|
|1||SH, WL||+1 NCLa, SH, WL, Urb, Rur||+1 or 2 NCLsa, SH, WL, Urb||+ NCLs, SH, WL, Urb, Rur|
|2||SH, WL||SH, WL||+1 NCLa, SH, WL, Urb, Rur||+1 or 2 NCLsa, SH, WL, Urb||+ NCLs, SH, WL, Urb, Rur|
|3||+1 HCL, SH, WL||+1 HCL, SH, WL||+1 NCLa, SH, WL, Urb, Rur||+1 or 2 NCLsa, SH, WL, Urb||+ NCLs, SH, WL, Urb, Rur|
|4||+1 or 2 HCLsb, SH, WL||+1 or 2 HCLsb, SH, WL||+1 NCLa or +2 HCLs, SH, WL, Urb, Rur||+1 or 2 NCLsa, SH, WL, Urb||+ NCLs, SH, WL, Urb, Rur|
|5||+ HCLs, SH, WL||+ HCLs, SH, WL||+1 NCLa and HCLs, SH, WL, Urb, Rur||+1 or 2 NCLsa and HCLs, SH, WL, Urb||+ NCLs, SH, WL, Urb, Rur|
Setting the WDFOVR code to 1 is the equivalent of disabling the override feature, so that each section's WDFEAS code alone determines the widening options which HERS will consider. This case is illustrated in the first row of Table 4-14. In this situation, if WDFEAS for a section is coded as 1, no widening is considered, while if WDFEAS equals 2, HERS will consider widening the shoulders and/or lanes. If WDFEAS is coded as 3, 4, or 5, HERS may also consider adding normal cost lanes, improving access control, and widening medians. When WDFEAS is coded as 3, HERS will only consider adding a lane when the existing facility has an odd number of lanes. When WDFEAS is coded as 4, HERS will consider adding one lane to a facility with an odd number of lanes, or adding two lanes to a facility with an even number of lanes.
Setting the WDFOVR code higher than 1 causes HERS to consider additional improvement options, including high cost lanes in some cases. Note that the "Rur" and "Urb" values in Table 4-14 are the same in each column. This occurs because WDFOVR is not used in assessing whether the median width and access control upgrades can be made to rural sections receiving additional lanes and substandard urban freeways undergoing reconstruction. Note also that HERS will not add lanes in excess of the MAXLNS value, regardless of how the WDFEAS or WDFOVR variables are coded.
When evaluating improvements, HERS typically uses the initial WDFEAS value at the beginning of the funding period to determine widening feasibility. However, when considering supplemental improvements, the WDFEAS value may first be adjusted downward. On rural sections receiving additional lanes and substandard urban freeways being reconstructed, the main improvement may consume all of the space available, and could preclude any additional upgrades to medians or access control that HERS might otherwise have considered. To address these situations, HERS evaluates supplemental upgrades based on a reduced WDFEAS value that factors in the effect of the main improvement on the initial WDFEAS value. The WDFEAS values shown in Table 4-14 represent the adjusted codes.
For example, a three-lane rural section with an initial WDFEAS of 3 might be resurfaced and have one lane added. Adding a lane would result in reducing WDFEAS to 1, so supplemental rural upgrades would not be considered. This reduction in WDFEAS values is why the "Rur" value doesn't appear in Table 4-14 in the column where WDFEAS equals 4. HERS only considers median width and access control upgrades to rural sections when lanes are added. If the WDFEAS value at the beginning of the funding period was 4, adding one or two lanes would reduce the WDFEAS code to 3 or 1, respectively. Therefore, for any case in which HERS would be considering median width and access control upgrades to rural sections, WDFEAS could not equal 4 (if the initial WDFEAS value was 5, it would remain 5 after adding lanes).
HERS updates WDFEAS in response to improvements on the section. See Table 4-8, "Widening Feasibility Code Adjustments," for the effects of improvements on WDFEAS.
For the 2002 C&P Report, the maximum number of lanes was set to 99 for all functional classes. The effect of setting MAXLNS to such a high number was to effectively eliminate it as a factor in regulating roadway width, leaving each section's WDFEAS value and the WDFOVR override value to determine widening limits. WDFOVR was set to 1, which precluded HERS from adding high cost lanes to any section.15
The HERS capacity model has two functions. The first is the calculation of section capacity; the second is the calculation of the number of lanes needed to accommodate the projected traffic volume in the design year (that is, how many additional lanes are needed).
HERS capacity routines are based upon Appendix N, "Procedures for Estimating Highway Capacity," of the HPMS Field Manual. Appendix N was revised in February 2002 and incorporates algorithms from the Highway Capacity Manual 200016.
For each section, HERS develops separate estimates of capacity for the peak and offpeak periods; and the peak-period estimates are developed separately for the peak direction and the opposite, or "counterpeak," direction. These three capacities are referred to as peak, counterpeak, and offpeak capacity. Differences in the three capacities result primarily from differences in the number of available travel lanes.
Corresponding to the three capacities, the three sets of lanes are referred to as peak, counterpeak, and offpeak lanes. Peak and counterpeak lanes represent lanes in one direction only; while offpeak lanes represent lanes in both directions.
Offpeak capacity is estimated using the coded value of the number of through lanes in the offpeak period; and peak capacity is estimated using the coded value of peak lanes. The latter value represents total through lanes for rural two- and three-lane roads and it represents peak-direction lanes for all other sections.
For sections with a peak-period directional factor that is less than 1.0, HERS obtained the number of counterpeak lanes by subtracting the number of peak lanes from the sum of offpeak lanes and an estimate of the number of extra travel lanes (if any) available during the peak period. For sections with surfaced shoulders, the maximum number of total peak-period lanes is assumed to equal twice the number of peak lanes. For curbed sections, the maximum number of extra peak-period lanes is assumed to equal the number of sides on which peak parking is not allowed. In both cases, the number of extra peak-period lanes is reduced, if necessary, to guarantee that the fraction of lanes in the counterpeak direction is no higher than the minimum number necessary to keep congestion in the counterpeak direction lower than congestion in the peak direction. Thus, the number of counterpeak lanes is never greater than the number of peak lanes, and it frequently is smaller.
For two-way sections, the three capacities also reflect three different directional factors. For peak capacity, the coded directional factor is used (this value represents the percentage of traffic flowing in the peak direction during the design hour). For counterpeak capacity, one minus this value is used. For offpeak capacity on two-way sections, more balanced traffic flow is assumed, and the "offpeak directional factor" is usually set to 0.5.17
HERS allows the user to specify whether to use the estimates of peak capacity produced by the above process or the values coded in the HPMS record. If the HPMS file has been processed by a version of the submittal software that uses capacity procedures based on the 1994 and 1997 HCM, the two sets of estimates will be essentially the same. If asked to use peak capacities that differ from those derived by the above procedure, then, for each section, HERS uses the ratio of the coded peak capacity to the internally derived peak capacity as a scale factor. This factor, referred to as the "capacity ratio" (CRATIO), is then used to adjust the internally derived estimates of offpeak and counterpeak capacity to produce values that are consistent with the coded value of peak capacity. That is, HERS uses CRATIO to ensure that the differential between the supplied and calculated capacity is carried through subsequent capacity calculations. If HERS is asked to use the internally generated capacities, the capacity ratios are set to 1.0.
17. The exception occurs for sections with an odd number of total lanes in the offpeak period. For these sections, the fraction of total lanes in the direction with the higher number of lanes is determined. The offpeak directional factor is then set to the lower of this value and the peak-period directional factor.
The capacity effects of any improvement that may affect capacity are estimated by:
- Using the capacity procedure to obtain new estimates of peak, offpeak, and counterpeak capacity; and
- Multiplying these values by the capacity ratio CRATIO to produce estimates that are consistent with the capacity values used prior to implementing the improvement.