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21 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 Figure 15. Selection of preferred design considerations, as they may influence drainage fea- pavement-type alternative(s). 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- · User costs. Alternatives with high user costs require special als considerations. evaluation, even if the overall life-cycle cost is low. High user · Continuity of adjacent pavements. When filling a gap costs indicate the potential for a high degree of user dis- between two similar pavement types, it may be preferable to satisfaction, or a negative impact on the traveling public. continue a similar pavement type to avoid a hopscotch pat- When high user costs are computed, the agency should tern and provide for continuity of maintenance operations review the project design and construction sequencing to and experience.
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22 · Continuity of adjacent lanes. Nonuniform sections can or may have noise duration issues due to longer periods of result in differential pavement performance and condition construction and/or M&R. across the width of the roadway. Consistent performance · Safety considerations. The particular characteristics of a across the width of the roadway is preferred. (The preferred wearing course surface, the need for delineation through uniformity is applicable to driving lanes only and not to pavement and shoulder contrast, reflectivity under high- existing shoulders that will remain shoulders.) way lighting, and the maintenance of a nonskid surface as · Traffic during construction. Speed of construction, accom- affected by the available materials may influence the pave- modating traffic during construction, safety to traffic during ment strategy selected in specific locations. In the context of construction, ease of replacement, anticipated future widen- nonskid surfaces, it is important to consider the profile and ing, seasons of the year when construction must be accom- texture durability (i.e., how long the desirable characteristics plished, and other related factors may have a strong influence are going to last). Excessive ruts on the surface often increase on the strategy selections in specific cases. Construction the likelihood of safety hazards such as hydroplaning, insuf- considerations can be especially important for the design ficient friction, and loss of control of the vehicle, especially of rehabilitation projects. For example, limited overhead in wet weather and at high speeds. clearances may preclude an overlay or limit its thickness · Subgrade soils. For new locations or reconstruction, the such that pavement-type selection is affected. Other geomet- ability of the foundation to support construction equipment ric factors, such as roadway width, guardrail heights, and and processes may be an important concern. Sometimes cutfill slopes often impact the design decision. it is necessary to stabilize subgrade soils with cementitious · Availability of local materials and experience. The avail- materials to provide a suitable working platform. Such sta- ability and adaptability of local material may influence the bilized subgrades often have not been considered as part selection of a pavement strategy. Also, the availability of of the pavement structure. The load-carrying capability of commercially produced mixes and the equipment capabil- a native soil, which forms the subgrade for the pavement ities of area contractors may influence the selection, partic- structure, is of paramount importance in pavement per- ularly on small projects. formance. Even for small projects, the inherent qualities · Conservation of materials/energy. Selection of a pavement of such native soils are far from uniform, and they are fur- strategy may be influenced by the criticality of materials sup- ther subjected to variations by the influence of weather. ply as well as by the energy requirements of materials produc- The characteristics of native soils not only directly affect the tion. The construction energy requirements associated with pavement structural design but also may, in certain cases, various pavement types may be an additional consideration. dictate the type of pavement economically justified for a · Local preference. The issues raised by consideration of given location. As an example, problem soils that change municipal or local government preferences and local indus- volume with time require a pavement structure able to con- tries may be outside the control of most highway engineers. form to seasonal variations in longitudinal and transverse However, the highway administrator often must take these profile. An approach sometimes used is to provide for staged preferences into consideration, especially if other factors do construction to accommodate large expected deformations not yield a clear pavement-type preference. over time. · Stimulation of competition. Most agencies consider it desir- · Experimental features. In some instances, it is necessary able to encourage improvements in products and methods to determine the performance of new materials or design through continued and healthy competition among the concepts by field testing under actual construction, envi- paving industries and materials suppliers. Where alternate ronmental, or traffic conditions. The incorporation of such pavement designs have comparable initial costs, includ- experimental features may dictate the strategy selected. ing the attendant costs of earthwork, drainage facilities, and · Future needs. Future needs on geometric or capacity other appurtenances, and provide comparable service-life or changes during the analysis period are evaluated to deter- life-cycle cost, the highway agency may elect to take alternate mine if the use of staged construction is warranted. bids to stimulate competition and obtain lower prices. · Maintenance capability. It is necessary to determine if the · Noise issues. Noise can have a significant impact on quality maintenance unit responsible for the pavement section has of life and is costly to mitigate after the fact. Tirepavement the experience and equipment to maintain all pavement noise mitigation is particularly important on urban high- alternatives being considered. ways. The life of the low-noise surface should be considered, · Sustainability. Sustainability in pavements is achieved as some deteriorate rapidly. Construction noise also can be through practices emphasizing energy efficiency, emissions an issue, influenced by factors such as the equipment type, reduction, and resource conservation. Incorporating these traffic rerouting, and day/night operations. So certain alter- strategies leads to an approach that balances environment natives may have noise intensity issues in sensitive settings conservation, societal needs, and economic development