National Academies Press: OpenBook

Guide for Pavement-Type Selection (2011)

Chapter: Chapter 5 - Selection of Preferred Pavement Alternatives

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Suggested Citation:"Chapter 5 - Selection of Preferred Pavement Alternatives." National Academies of Sciences, Engineering, and Medicine. 2011. Guide for Pavement-Type Selection. Washington, DC: The National Academies Press. doi: 10.17226/14538.
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Suggested Citation:"Chapter 5 - Selection of Preferred Pavement Alternatives." National Academies of Sciences, Engineering, and Medicine. 2011. Guide for Pavement-Type Selection. Washington, DC: The National Academies Press. doi: 10.17226/14538.
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Suggested Citation:"Chapter 5 - Selection of Preferred Pavement Alternatives." National Academies of Sciences, Engineering, and Medicine. 2011. Guide for Pavement-Type Selection. Washington, DC: The National Academies Press. doi: 10.17226/14538.
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Suggested Citation:"Chapter 5 - Selection of Preferred Pavement Alternatives." National Academies of Sciences, Engineering, and Medicine. 2011. Guide for Pavement-Type Selection. Washington, DC: The National Academies Press. doi: 10.17226/14538.
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Suggested Citation:"Chapter 5 - Selection of Preferred Pavement Alternatives." National Academies of Sciences, Engineering, and Medicine. 2011. Guide for Pavement-Type Selection. Washington, DC: The National Academies Press. doi: 10.17226/14538.
×
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Suggested Citation:"Chapter 5 - Selection of Preferred Pavement Alternatives." National Academies of Sciences, Engineering, and Medicine. 2011. Guide for Pavement-Type Selection. Washington, DC: The National Academies Press. doi: 10.17226/14538.
×
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Suggested Citation:"Chapter 5 - Selection of Preferred Pavement Alternatives." National Academies of Sciences, Engineering, and Medicine. 2011. Guide for Pavement-Type Selection. Washington, DC: The National Academies Press. doi: 10.17226/14538.
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20 5.1 Overview Upon completion of the LCCA, the alternatives are evalu- ated using economic and noneconomic factors to select the preferred pavement type(s). The outcome of this process is a single preferred pavement type for traditional design-bid-build projects and multiple preferred pavement types for alternative bid projects, which are consistent with the agencies financial goals, policy decisions, and experience. Figure 15 presents a flow chart of the proposed approach for selecting preferred pavement types, and this chapter presents a detailed discussion. The selection process is outlined as follows: 1. The alternatives are evaluated using the economic factors; alternatives that fail to meet the economic criteria are eliminated. 2. The alternatives that meet the economic criteria are eval- uated using the noneconomic factors. • If an alternative fails to meet the noneconomic criteria, further evaluation may be necessary to ascertain whether the noneconomic factors unduly override its inclusion. If the risks from noneconomic factors outweigh the eco- nomic advantages, the alternative is eliminated. • If there are no noneconomic factors to override its inclu- sion, the alternative is selected as a qualified alternative. 3. The alternatives that meet both economic and noneconomic criteria are considered as qualifying alternatives. • If there is only a single qualifying alternative, it is selected as the most-preferred alternative. 4. When there are two or more qualified alternatives, then the economic and noneconomic aspects of these alternatives are weighed using an alternative-preference screening matrix to identify the most preferred type. The screening matrix is used to evaluate if there are considerable differences among the alternatives. • If there is a clear cut preference among the alternatives, the most advantageous alternative is recommended for selection. • Conversely, if the differences between all or some of the alternatives are not significant, then the similar alter- natives could be considered for alternate pavement- type bidding. 5.2 Economic Selection Factors An important step in the selection of the preferred strategy is the consideration of the financial aspects of the project. The agency evaluates the pavement-type alternatives on the basis of these aspects and their importance. The following list describes the economic factors that should be included in the evaluation: • Initial costs. Agencies may set maximum funding levels for individual projects so that the entire system can be maintained at a desired level. Such constraints may result in eliminating some alternatives, particularly those with high initial costs, even if the alternatives are attractive from a life-cycle cost perspective. The evaluation should deter- mine if the first costs of an alternative exceed the available resources or would impact the management of the overall system. • Rehabilitation costs. Certain alternatives may provide a low overall life-cycle cost but require several rehabilitation activities to maintain the desired functional and structural performance level. Such costs may have an impact on the management of the entire system. Frequent interventions also may result in higher work zone user costs and impacts on local business and community. The evaluation should determine if an alternative that requires frequent rehabili- tation actions may be suitable for the project. • Maintenance costs. Certain alternatives may require a dis- proportionate maintenance effort over their lifetime that exceeds the resources available for applying the mainte- nance. The evaluation should focus on the maintenance actions that an alternative may require to maintain per- formance levels over its life. C H A P T E R 5 Selection of Preferred Pavement Alternatives

determine if the impacts to the user can be reduced. In cases where high user costs cannot be reduced, consideration should be given to alternatives with a lesser impact. More information on user costs may become available from FHWA, NCHRP, and state departments of transportation (DOTs) research efforts. • Life-cycle costs. The life-cycle costs indicate the aggregation of estimated initial and future costs normalized to their time value. If the percent difference between the two lowest cost strategies is greater than some established threshold require- ment, then the lowest cost strategy is accepted as the most economical one. If, in contrast, the percent difference is less than this threshold requirement, then the life-cycle costs of the two strategies are deemed equivalent, thereby leav- ing the analyst with the option of reevaluating the strate- gies or allowing other factors to dictate the strategy selection process. The percent difference threshold value between two competing alternatives will depend on the accuracy of the factors collected by the agency. The percent difference, typ- ically 5 to 20 percent, should be determined and set by the pavement-type selection committee. Where data is avail- able, the impact of the variances of the LCCA input variables on the variance of the NPV should be considered in estab- lishing the percent difference threshold value. 5.3 Noneconomic Selection Factors In addition to economic factors, numerous noneconomic factors must be considered in making a pavement-type selec- tion for a specific project. The importance of these factors may vary from project to project. The following list describes the factors that should be included in pavement-type selection. This is not an exhaustive list; other factors and project-specific conditions should be considered as necessary. • Roadway/lane geometrics. Lane widths may be fixed by design standards, yet there will be occasions, especially with rehabilitation design, when it is necessary to work with vary- ing widths. Lane widths also play a major role in where wheel loads will be located. Overall, lane width can be important in determining the width and type of shoulder, as well as the type of pavement. Longitudinal grades and the absence or presence of vertical curves can be important pavement design considerations, as they may influence drainage fea- tures and even the type and speed of traffic to use the facil- ity. Slower traffic produces larger deformations, stresses, and strains in a pavement structure and requires special materi- als considerations. • Continuity of adjacent pavements. When filling a gap between two similar pavement types, it may be preferable to continue a similar pavement type to avoid a hopscotch pat- tern and provide for continuity of maintenance operations and experience. 21 Figure 15. Selection of preferred pavement-type alternative(s). • User costs. Alternatives with high user costs require special evaluation, even if the overall life-cycle cost is low. High user costs indicate the potential for a high degree of user dis- satisfaction, or a negative impact on the traveling public. When high user costs are computed, the agency should review the project design and construction sequencing to

• Continuity of adjacent lanes. Nonuniform sections can result in differential pavement performance and condition across the width of the roadway. Consistent performance across the width of the roadway is preferred. (The preferred uniformity is applicable to driving lanes only and not to existing shoulders that will remain shoulders.) • Traffic during construction. Speed of construction, accom- modating traffic during construction, safety to traffic during construction, ease of replacement, anticipated future widen- ing, seasons of the year when construction must be accom- plished, and other related factors may have a strong influence on the strategy selections in specific cases. Construction considerations can be especially important for the design of rehabilitation projects. For example, limited overhead clearances may preclude an overlay or limit its thickness such that pavement-type selection is affected. Other geomet- ric factors, such as roadway width, guardrail heights, and cut–fill slopes often impact the design decision. • Availability of local materials and experience. The avail- ability and adaptability of local material may influence the selection of a pavement strategy. Also, the availability of commercially produced mixes and the equipment capabil- ities of area contractors may influence the selection, partic- ularly on small projects. • Conservation of materials/energy. Selection of a pavement strategy may be influenced by the criticality of materials sup- ply as well as by the energy requirements of materials produc- tion. The construction energy requirements associated with various pavement types may be an additional consideration. • Local preference. The issues raised by consideration of municipal or local government preferences and local indus- tries may be outside the control of most highway engineers. However, the highway administrator often must take these preferences into consideration, especially if other factors do not yield a clear pavement-type preference. • Stimulation of competition. Most agencies consider it desir- able to encourage improvements in products and methods through continued and healthy competition among the paving industries and materials suppliers. Where alternate pavement designs have comparable initial costs, includ- ing the attendant costs of earthwork, drainage facilities, and other appurtenances, and provide comparable service-life or life-cycle cost, the highway agency may elect to take alternate bids to stimulate competition and obtain lower prices. • Noise issues. Noise can have a significant impact on quality of life and is costly to mitigate after the fact. Tire–pavement noise mitigation is particularly important on urban high- ways. The life of the low-noise surface should be considered, as some deteriorate rapidly. Construction noise also can be an issue, influenced by factors such as the equipment type, traffic rerouting, and day/night operations. So certain alter- natives may have noise intensity issues in sensitive settings or may have noise duration issues due to longer periods of construction and/or M&R. • Safety considerations. The particular characteristics of a wearing course surface, the need for delineation through pavement and shoulder contrast, reflectivity under high- way lighting, and the maintenance of a nonskid surface as affected by the available materials may influence the pave- ment strategy selected in specific locations. In the context of nonskid surfaces, it is important to consider the profile and texture durability (i.e., how long the desirable characteristics are going to last). Excessive ruts on the surface often increase the likelihood of safety hazards such as hydroplaning, insuf- ficient friction, and loss of control of the vehicle, especially in wet weather and at high speeds. • Subgrade soils. For new locations or reconstruction, the ability of the foundation to support construction equipment and processes may be an important concern. Sometimes it is necessary to stabilize subgrade soils with cementitious materials to provide a suitable working platform. Such sta- bilized subgrades often have not been considered as part of the pavement structure. The load-carrying capability of a native soil, which forms the subgrade for the pavement structure, is of paramount importance in pavement per- formance. Even for small projects, the inherent qualities of such native soils are far from uniform, and they are fur- ther subjected to variations by the influence of weather. The characteristics of native soils not only directly affect the pavement structural design but also may, in certain cases, dictate the type of pavement economically justified for a given location. As an example, problem soils that change volume with time require a pavement structure able to con- form to seasonal variations in longitudinal and transverse profile. An approach sometimes used is to provide for staged construction to accommodate large expected deformations over time. • Experimental features. In some instances, it is necessary to determine the performance of new materials or design concepts by field testing under actual construction, envi- ronmental, or traffic conditions. The incorporation of such experimental features may dictate the strategy selected. • Future needs. Future needs on geometric or capacity changes during the analysis period are evaluated to deter- mine if the use of staged construction is warranted. • Maintenance capability. It is necessary to determine if the maintenance unit responsible for the pavement section has the experience and equipment to maintain all pavement alternatives being considered. • Sustainability. Sustainability in pavements is achieved through practices emphasizing energy efficiency, emissions reduction, and resource conservation. Incorporating these strategies leads to an approach that balances environment conservation, societal needs, and economic development 22

in existing practices. Sustainable practices include increased use of recycled materials, industrial by-products, and local materials; decreased use of energy-intensive materials and construction processes; improvements in material produc- tion and processes; techniques that preserve or increase the longevity of pavements; and eco-friendly design alternatives. Life-cycle assessment (LCA) methods typically are used in evaluating the environmental impact of materials, equip- ment, and processes used in pavements. The LCA-based environmental impacts can be incorporated qualitatively or quantitatively in the pavement-type selection process. 5.4 Weighing of Economic and Noneconomic Factors Using Alternative-Preference Screening Matrix The pavement-type selection process should weigh both economic and noneconomic factors to ensure that the agency goals and policies are incorporated in decision making. An alternative-preference screening matrix is suggested for this purpose. The screening matrix is a decision support tool that is designed to help agencies determine whether there are advan- tages in selecting one alternative over others and whether these alternatives should be evaluated more closely. The following sections describe how to set up the screening matrix and evaluate the results obtained. Appendix A illus- trates the application of the screening matrix and includes an example of its use. Step 1: Identify and Group Evaluation Factors The economic and noneconomic factors that have a potential impact on the pavement-type selection process for a given project are identified and grouped. The factors identified in Sections 5.2 and 5.3 are sug- gested. A suggested grouping structure and sample factors are illustrated in Table 1. The factor groups could include economic factors, construction fac- tors, local factors, maintenance factors, traffic and safety factors, environmental factors, and others. Agencies are expected to modify the contents of Table 1 as necessary to best suit their goals, expecta- tions, and project requirements. The evaluation fac- tors and groups may vary from project to project within an agency. Step 2: Assign Group and Individual Factor Weights Next, weights must be assigned to each of the factor groups and each factor within a group to reflect their importance to the pavement-type selection process for a given project. Table 2 and Table 3 illustrate the group and factor weighing scheme, respectively. The factor groups and factors within a group can be assigned equal or unequal weights, but the sum of all group weights and all the factor weights within each group must equal 100 percent. Step 3: Assign Preference Rating of Individual Factors To facilitate a comparative evaluation of alternatives, the evaluation factors are assigned with preference 23 Table 1. Grouping structure of the alternative- preference screening matrix. elpmaxEnoisreVlareneG Group A Factor A1 Factor A2 … Factor An Economic factors Initial costs Future rehabilitation costs … User costs Group B Factor B1 Factor B2 … Factor Bn Construction factors Continuity of adjacent lanes Traffic during construction … Lane geometrics Group C Factor C1 Factor C2 … Factor Cn Local factors Availability of local materials District/local preferences … Stimulation of competition Group D Factor D1 Factor D2 … Factor Dn Other factors Noise Subgrade soils ... Experimental features Table 2. Group weights of the screening matrix. elpmaxEnoisreVlareneG Group Weight Group Weight A WA% Economic factors 50% B WB% Construction factors 25% C WC% Local factors 10% D WD% Other factors 15% Total score = 100% Total score = 100% Note: W = weight. Table 3. Factor weights of the screening matrix. elpmaxEnoisreVlareneG Group A Weight Economic Factors Weight A1 WA1 %03stsoclaitinI% A2 WA2% Future rehabilitation costs 25% A3 WA3% Road-user costs 20% An WAn% Future maintenance costs 25% Group total = 100% =latotpuorG 100%

ratings using predetermined criteria. The purpose of the ratings is to quantify the relative advantages and disadvantages among the alternatives for each evalu- ation factor. When an alternative offers significant advantages associated with a given evaluation factor, then the alternative is rated with a high preference for that factor. The rating scheme can be discrete or continuous. While a discrete rating scheme is simple to use, a con- tinuous rating scheme provides more flexibility for users. As with factors, groups, and weights, it is pro- posed that the agencies develop their own rating guide- lines that reflect their goals and expectations. As a first step, each agency should ask decision makers about the factors they currently use in making pavement-type decisions and what additional factors should be con- sidered in the process. Test runs should be made on several older projects to determine if the proposed screening process results in acceptable pavement selec- tions. The pavement-type selection committee can help establish these guidelines for the agency’s use. Table 4 provides sample guidelines on rating indi- vidual factors on a discrete scale. For example, the Initial Cost factor is assigned a preference rating of “high” when the initial cost value of an alternative is within a 5 percent difference of the lowest values of all candidates or “low” if the initial cost difference of the alternative exceeds 10 percent of the lowest value. Step 4: Score Pavement-Type Alternatives Upon assigning preference ratings, the numerical weighted scores of evaluation factors and groups are calculated for each alternative. Ratings of “low,” “medium,” and “high,” if used, should be converted to numerical scores. Table 5 presents example crite- ria for converting these ratings to a numerical scale. 24 Table 4. Sample rating guidelines for the alternative-preference screening matrix. Table 5. Example criteria for preference rating. Factor Low Medium High Initial costs Cost >10% Cost >5% and <10% Cost within 5% Life-cycle costs Cost >20% Cost >10% and <20% Cost within 10% User costs User cost >20% User cost >10% and <20% User cost within 10% Future rehabilitation costs Cost >10% Cost >5% and <10% Cost within 5% Future maintenance costs Cost >10% Cost >5% and <10% Cost within 5% Roadway/lane geometrics Significant complexity to accommodate Moderate complexity to accommodate Easy to accommodate Continuity of adjacent pavements Significant issues Some significant issues possible No significant issues Continuity of adjacent lanes Significant issues Some significant issues possible No significant issues Availability of local materials and experience Lack of local experience Some experience Commonly used Traffic during construction Very difficult to accommodate Somewhat difficult to accommodate Easy to accommodate Noise Much higher noise likely Some increased noise possible No difference in noise generated Subgrade soils Significant issues for construction Some issues possible for construction No significant issues for construction District/local preference No preference Some preference Significant preference Safety considerations Significant issues related to safety features Some issues related to safety features Better safety features Conservation of materials/energy Much higher materials/ energy use Somewhat higher materials/energy use No significant difference Stimulation of competition Very few capable contractors Some experience Common experience for each Maintenance capability Little to no local experience Some experience Common experience for each Future needs Very difficult to accommodate Somewhat difficult to accommodate Easy to accommodate Experimental features Common technology National but no local experience New and unproven technology Preference Rating Numerical Score No difference 0% Low 20% Medium low 40% Medium 60% Medium high 80% High 100%

For a given alternative, the numerical scores of each evaluation factor are multiplied by their correspon- ding factor weights to calculate the weighted scores of factors. The sum of weighted scores of factors within each group is the unweighted score of that group. The example in Table 6 calculates the weighted score for individual factors within the Economic Factors group and the unweighted score for that group. The weighted group scores are then calculated by multiplying their unweighted score by their corresponding group weights (see Table 7). The sum of weighted group scores is the total score for that alternative; it should not exceed 100 percent. Step 5: Interpret Results Based on the final scores of alternatives, the “best pos- sible” pavement-type alternatives are selected. When the final score of an alternative is higher than that of other candidates, the alternative with the high- est score may be much better suited than others. How- ever, when the final scores of multiple alternatives are comparable, any of these alternatives could be selected. Such cases are well suited for alternate bidding. If no alternative appears to be satisfactory, further investi- gation is needed. Agencies should determine their own criteria to interpret the screening-matrix results. An agency can develop a threshold value to determine how different the alternatives are. For instance, if the difference in the final scores of two alternatives is more than 10, the alternative with the higher score can be selected as the preferred one. Recognizing that the project goals and the choice of feasible alternatives are unique to each project, this guide recommends the application of informed judg- ment and agency experience in the selection process, with or without a threshold criterion in place. The screening matrix provides a systematic framework for practical decision making by setting “musts” and “wants” of an ideal choice for the project, exploring and prioritizing alternatives based on their strengths and weaknesses, and choosing the most-preferred alternative(s). Table 8 provides a template of the screening matrix. Users can add or eliminate any number of alternatives, groups, and individual factors within a group, as appropriate. Appendix A illustrates the application of the alternative-preference screening matrix. 25 Table 6. Example of the calculation of weighing scores for individual factors. Table 7. Example of the calculation of weighted group scores. Economic Factors Individual Factor Weight Preference Rating Numerical Rating Weighted Score Initial costs 30% Medium 60% 18.0% Future rehab costs 25% High 100% 25.0% User costs 20% Low 20% 4.0% Future maintenance costs 25% Medium- low 40% 10.0% Total unweighted score for Economic Factors %75 Group Group Weight Unweighted Group Score Weighted Group Score Economic factors 50% 57% 28.5% Construction factors 25% 45% 11.3% Local factors 10% 25% 2.5% Other factors 15% 15% 2.3% %6.44xirtamehtfoerocslatoT

26 Factor Factor Weight Alternative 1 (Alt1) Alternative 2 (Alt2) Rating Weighted Score Rating Weighted Score ApuorG Factor A1 WA1 RA1-Alt1 WA1*RA1-Alt1 RA1-Alt2 WA1 * RA1-Alt2 Factor A2 WA2 RA2-Alt1 WA2 *RA2-Alt1 RA2-Alt2 WA2 *RA2-Alt2 Factor A3 WA3 RA3-Alt1 WA3 *RA3-Alt1 RA3-Alt2 WA3 *RA3-Alt2 Factor An WAN RAN-Alt1 WAN *RAN-Alt1 RAN-Alt2 WAn *RAN-Alt2 Group A unweighted total 100% Σ(WAi*RAi-Alt1) Σ(WAi * RAi-Alt2) Group B Factor B1 WB1 RB1-Alt1 WB1*RB1-Alt1 RB1-Alt2 WB1*RB1-Alt2 Factor B2 WB2 RB2-Alt1 WB2 *RB2-Alt1 RB2-Alt2 WB2 *RB2-Alt2 Factor B3 WB3 RB3-Alt1 WB3 *RB3-Alt1 RB3-Alt2 WB3 *RB3-Alt2 Factor Bn WBN RBN-Alt1 WBN *RBN-Alt1 RBN-Alt2 WBn *RBN-Alt2 Group B unweighted total 100% Σ(WBi*RBi-Alt1) Σ(WBi*RBi-Alt2) Group C Factor C1 WC1 RC1-Alt1 WC1*RC1-Alt1 RC1-Alt2 WC1*RC1-Alt2 Factor C2 WC2 RC2-Alt1 WC2 *RC2-Alt1 RC2-Alt2 WC2 *RC2-Alt2 Factor C3 WC3 RC3-Alt1 WC3 *RC3-Alt1 RC3-Alt2 WC3 *RC3-Alt2 Factor Cn WCN RCN-Alt1 WCN *RCN-Alt1 RCN-Alt2 WCn*RCN-Alt2 Group C unweighted total 100% Σ(WCi*RCi-Alt1) Σ(WCi*RCi-Alt2) Group D Factor D1 WD1 RD1-Alt1 WD1*RD1-Alt1 RD1-Alt2 WD1*RD1-Alt2 Factor D2 WD2 RD2-Alt1 WD2 *RD2-Alt1 RD2-Alt2 WD2 *RD2-Alt2 Factor D3 WD3 RD3-Alt1 WD3 *RD3-Alt1 RD3-Alt2 WD3 *RD3-Alt2 Factor Dn WDN RDN-Alt1 WDN *RDN-Alt1 RDN-Alt2 WDn*RDN-Alt2 Group D unweighted total 100% Σ(WDi*RDi-Alt1) Σ(WDi*RDi-Alt2) Subtotals Group Weights Group Unweighted Total Group Weighted Total Group Unweighted Total Group Weighted Total Group A WA Σ(WAi*RAi-Alt1) WA *Σ(WAi*RAi-Alt1) Σ(WAi*RAi-Alt2) WA *Σ(WAi*RAi-Alt2) Group B WB Σ(WBi*RBi-Alt1) WB *Σ(WBi*RBi-Alt1) Σ(WBi*RBi-Alt2) WB *Σ(WBi*RBi-Alt2) Group C WC Σ(WCi*RCi-Alt1) WC *Σ(WCi*RCi-Alt1) Σ(WCi*RCi-Alt2) WC *Σ(WCi*RCi-Alt2) Group D WD Σ(WDi*RDi-Alt1) WD *Σ(WDi*RDi-Alt1) Σ(WDi*RDi-Alt2) WD *Σ(WDi*RDi-Alt2) Grand total 100% Final score- Alt 1 Final score- Alt 2 Table 8. Alternative-preference screening-matrix worksheet.

Next: Chapter 6 - Alternate Pavement-Type Bidding »
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 703: Guide for Pavement-Type Selection includes processes for conducting systematic evaluations of pavement alternatives and for making decisions on pavement-type selection.

The processes may be used for both agency-based and contractor-based type selections and may be applied to different pavement types and structures.

Further elaboration on the work performed in developing this report is available online.

In July 2013, the following errata on NCHRP Report 703 was issued: On page 67, in the second bullet point at the bottom of the page, the second to last sentence should read, “To maximize the economic value, the agency should consider alternatives that stimulate competition and incorporate innovative approaches.” The wording has been corrected in the online version of the report.

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