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7 Chapter 3. Process for 3R Project Development This chapter presents the process for development of 3R projects including the identification of candidate 3R projects and the role of safety and traffic operational considerations in design decisions. 3.1 How Does the Design Process for 3R Projects Differ from the Design Process for New Construction and Reconstruction Projects? The current design process for new construction projects is based primarily on the dimensional design criteria presented in the AASHTO Green Book (4) and in the design policies of individual highway agencies. It is appropriate to use established dimensional design criteria for new construction projects because, in such projects, there is no existing roadway with a safety and traffic operational performance history that can be used to guide the design process. Established dimensional design criteria provide an aspirational goal for design of reconstruction projects. Where a roadway is being fully reconstructed, design improvements may be feasible with limited additional cost, except where such improvements would substantially impact adjacent development, established communities, or sensitive environments; in these situations, highway agencies typically seek a design exception to minimize such impacts. 3R projects are usually initiated based on the need for pavement resurfacing and are most appropriately considered as maintenance activities. A performance-based design process provides the basis for design of 3R projects focusing on the decision about which projects should be resurfaced without accompanying design improvements and which projects should have design improvements incorporated. The design process for 3R projects begins with the recognition that the project will be implemented on an existing road whose past safety and traffic operational performance is known and should serve as a key factor in design decisions. Unlike new construction and reconstruction projects, which are designed with reference to dimensional design criteria presented in the AASHTO Green Book (4), these guidelines do not establish dimensional design criteria for 3R projects. Research has found that any set of dimensional design criteria for 3R projects is likely to produce suboptimal results in terms of crash frequency and severity reduction (7). Rather, 3R design decisions should be based on an assessment of the safety and traffic operational performance of the existing road and the cost-effectiveness of potential design improvements. This approach is most likely to maximize the crash frequency and severity reduction that can be obtained within any budget level for 3R projects. Geometric design improvements should be considered as part of a 3R project in the following situations: ï· An analysis of the crash history of the existing road identifies one or more crash patterns that are potentially correctable by a specific design improvement, or
8 ï· An analysis of the traffic operational level of service (LOS) indicates that the LOS is currently lower than the highway agencyâs target LOS for the facility or will become lower than the target LOS within the service life of the planned pavement resurfacing (typically 7 to 12 years), or ï· A design improvement would be expected to reduce sufficient crashes over its service life to be cost-effective; i.e., the anticipated crash reduction benefits over the service life of the project should exceed the improvement implementation cost. Procedures for evaluating the cost-effectiveness of design improvements are presented in Chapter 5 of these guidelines. In the absence of any of the three situations defined above, there is no indication that a design improvement is needed as part of a 3R project, and the existing roadway and roadside geometric features should remain in place. It makes little sense to invest scarce resources in design improvements as part of a 3R project where the existing roadway is performing well and where potential design improvements would not be cost-effective; the funds needed for such a project can be better invested in projects that do address documented performance concerns or where potential design improvements would be cost-effective. In particular, improvement of systemwide safety across the road network is so important that funds invested with the objective of improving safety should be directed toward projects where it can be demonstrated that substantial safety benefits will likely be obtained. One potential exception to the guidance given above occurs if the normal pavement cross slope for an existing roadway to be resurfaced is less than the dimensional design criteria for normal cross slope in the AASHTO Green Book (4) or the design policy of the responsible highway agency. For, example, the AASHTO Green Book recommends a normal cross slope of 1.5 to 2 percent for paved roadways. There are no existing cost-effectiveness analysis procedures to address the selection of normal cross slope values, but appropriate pavement cross slope is needed for proper drainage during precipitation and should be provided. Therefore, it is recommended that normal pavement cross slope be increased to the criteria applicable to new construction and reconstruction as part of pavement resurfacing, whenever practical. This guidance applies only to normal cross slope, but does not apply to superelevated horizontal curves. The pavement cross slope on superelevated curves for rural two-lane and multilane highways is addressed by a CMF in the AASHTO Highway Safety Manual (2). Therefore, the need for superelevation improvement/restoration for horizontal curves on these facility types can be considered through cost-effectiveness analyses. The reliance on cost-effectiveness to guide design decisions for 3R projects has several advantages: ï· Highway agencies can have confidence that funds invested in design improvements intended to reduce crashes as part of 3R projects are, in fact, likely to result in reduced crashes. ï· Since crash frequency for a road generally increases with increasing traffic volume, the use of cost-effectiveness analysis as a basis for design decisions means that the likelihood of design improvements being included in a 3R project increases with increasing traffic
9 volume. This dependence of design decisions on traffic volume levels is logical and desirable and is not fully reflected in most current dimensional design criteria for new construction and reconstruction. ï· A cost-effectiveness approach will focus improvement needs on low-cost improvements with documented safety effectiveness, which are most consistent with the limited scope of 3R projects. However, the procedures are flexible enough that higher cost improvements can be considered where benefits are sufficient to justify their implementation. If extensive geometric improvements are found to be cost-effective, consideration may be given to reclassifying the project as a reconstruction project. Section 4.4 demonstrates that reliance on dimensional design criteria will provide suboptimal results, with some investments made at locations where they are not cost-effective and other investments not made at locations where they would be cost-effective. 3.2 How Should Candidate 3R Projects Be Identified? Candidate 3R projects should be identified primarily on the basis of pavement condition and the need for pavement resurfacing. Most highway agencies assess pavement condition and establish resurfacing priorities with the assistance of pavement management systems. Resurfacing at the proper time in the life of the pavement prolongs the life of the pavement and avoids more costly repairs. There are essentially three approaches that may be used in pavement management for project and treatment selection under fiscally constrained situations: ranking, prioritization, and optimization. Each of these three analysis approaches provides a method for identifying an optimal strategy for preserving the condition of the network, given any constraints that may exist. One of the easiest methods for selecting projects is to rank needs based on some type of agency priority, such as pavement condition or traffic levels, or both. A common method of ranking needs is to list road sections in sequential order by pavement condition rating, and to fund projects with the worst pavement condition until the available funding limits have been met. This approach is commonly referred to as a âworst-firstâ strategy in which the pavements in the worst condition are assigned the highest priority for funding. The next level of sophistication in project and treatment selection is a prioritization process, in which the most cost-effective use of available funding over the analysis period is identified. This approach is preferred over a ranking approach because consequences of delaying or accelerating a treatment are evaluated, and the cost-effectiveness of a treatment is taken into account in developing the program recommendations. An optimization analysis is a more complex analysis to determine how to efficiently allocate resources so that network conditions are maximized and costs are minimized. In some cases, agencies add additional constraints to help meet agency objectives. For example, an additional constraint that prevents any interstate pavement from dropping below a particular condition level
10 may be included as an analysis parameter. An advantage of the optimization analysis is the ability to incorporate risk into the analysis. One of the challenges associated with the use of pure optimization is the difficulty in linking network level results with project-level results. At the network level, the results of an optimization analysis typically provide the percentage of the road network that should be moved from one condition state to another through maintenance or rehabilitation actions. While most candidate 3R projects are identified based on the need for pavement resurfacing, any project that does not involve new construction or reconstruction can be addressed with the design guidelines in this document. For example, projects on existing roads identified with crash analysis tools, congestion management systems, or any other tool can be designed as a 3R project so long as the scope of the project does not fall within the definition of reconstruction, as presented in Section 2.1.2. Thus, highway agencies have substantial flexibility to identify candidate 3R projects in any manner, so long as the resulting project falls within the definition of a 3R project. Most 3R projects, even if they are not initiated originally by pavement resurfacing needs, are scheduled for implementation in conjunction with pavement resurfacing to obtain economies of scale. However, a design improvement that falls within the definition of 3R work can be designed and implemented in accordance with the guidelines presented in this document, regardless of whether pavement resurfacing is performed as part of the project. 3.3 Assessment of Needs for Improvements in Addition to Resurfacing As noted earlier, there are three types of assessments that are generally conducted to investigate whether a design improvement is appropriate in conjunction with pavement resurfacing. These are: ï· Crash history review and analysis ï· Traffic operational analysis ï· Cost-effectiveness analysis All three of these assessment approaches should be applied to every candidate 3R project. In some cases, one or more of these assessments can be applied very quickly. For example, if the crash history for a candidate project site includes no crashes, no crash history review is needed; if the candidate project site has experienced very few crashes, the crash history review should require only a limited effort. If a candidate project has a very low annual average daily traffic volume (AADT), a quick traffic operational review may be all that is needed to establish that the LOS of the project site is acceptable. Conversely, sites with high crash frequencies, or high traffic volumes, will require, and deserve, more involved analyses. Each of these assessment types is discussed below.
11 3.3.1 Crash History Review A crash history review should be a routine part of the design of every 3R project. Crash history data for a period of at least three, and preferably five, years should be reviewed to identify existing crash patterns and assess the need for design improvements. Depending upon the length and complexity of the project and the number of crashes that have occurred during the assessment period, the crash history review may involve: ï· review of automated crash data and/or hard copy police crash reports ï· tabulation of crash severities and types overall and by location within the project, including comparison to average crash characteristics for similar corridors ï· preparation of collision diagrams ï· comparison of intersections and segments within the project limits to average crash frequencies of similar intersections and segments for the same facility type The scope of the crash history review is flexible and will vary from project to project. If a candidate 3R project has experienced no crashes in the assessment period, no further crash review is needed and the assessment can move on to traffic operational assessment and cost- effectiveness analysis (see below). If only a few crashes have occurred, the crash history review can be accomplished quickly by reviewing automated crash data and/or hard copy police crash reports. The assessment should focus on determining whether there are repeated crashes of any given type or at any given location that are indicative of a crash pattern and that are potentially correctable by a design improvement. If a substantial number of crashes have occurred in the assessment period, then more effort will need to be devoted to the review, because the potential for crash patterns to be identified is greater. Tabulations of crash severities and types, overall and by location, should be prepared and collision diagrams may be useful in identifying crash patterns. Table 1 shows an example from a report in the AASHTOWare Safety Analyst (9,10) software tools illustrating the format of crash tabulations that may be useful in crash history review.
Ta Collision collision showing straight a crashes i shows th direction Chapter 5 Safety An typical co crashes, a locations the prepa The crash priority t damage-o serious in Chapter 5 diagnosin crash tab HSM Ch tests. ble 1. Exam diagrams il of the vehic the intended head, turnin s illustrated e classificati ; angle; or re . The prepa alyst softwa llision diag s shown in as well. In p ration of co history rev o address in nly or mino jury crashe of the AAS g crash patt ulations, sta apter 5 enab ple Crash T lustrate the les involved direction o g left, turnin by the orien on of a cras ar end. Proc ration of co re tools and ram. Collisi the figure, b articular, th llision diagr iew should ongoing saf r-injury cra s should rec HTO Highw erns at parti tistical tests les the inve ype Summ Soft crash locatio in each cra f travel for e g right, or b tation at wh h as head-on edures for p llision diagr with other on diagrams ut collision e AASHTO ams for non focus on fata ety program shes are not eive priority ay Safety M cular sites. H , and collisio stigator to re 12 ary from th ware (9,10) n and the pr sh. Pre-crash ach vehicle acking. Ma ich the pre-c ; sideswipe reparing co ams can be a commerciall are most co diagrams ca Ware Safety -intersection l and seriou s. Crash pat unimportan in program anual (2) p SM Chapte n diagrams view quanti e AASHTO e-crash mov movement and whethe nner of colli rash movem , opposite di llision diagr utomated w y available mmonly us n be prepare Analyst so locations. s injury cra terns that co t, but sites w ming design rovides a go r 5 includes in diagnosis tative result Ware Safe ements and s are illustra r the vehicle sion for mu ent arrows rection; side ams are pre ith the AAS software. Fi ed to evalua d for non-in ftware tools shes, which nsist prima ith patterns improveme od guide to guidance o of crash pa s and apply ty Analyst manner of ted with arr was going ltiple-vehicl intersect and swipe, sam sented in HS HTOWare gure 1 show te intersecti tersection can automa are the high rily of prope of fatal or nts. the process n the use of tterns. Use quantitative ows e e M s a on te est rty- of of
Figure 1. Example collision diagra Soft 13 m from the ware (9,10) AASHTOW are Safety Analyst
14 There is no specific minimum number of crashes that constitutes a crash pattern. Identification of crash patterns depends on the context of particular sites. Engineering judgment based on highway agency experience with particular types of sites may be the best guide in identification of crash patterns. One approach that is potentially applicable to some types of sites is to apply the testing procedures in HSM Chapter 5 to determine whether crash experience at a given site exceeds the expected crash experience for sites of that particular type and traffic volume level. HSM Chapter 6 explains how crash patterns can be used in identifying and selecting particular countermeasure types with applications likely to reduce particular crash types. The concepts presented in HSM Chapters 5 and 6 have been automated in the AASHTOWare Safety Analyst software, which includes safety management tools for application to specific highway types. Use of Safety Analyst requires initial effort to organize a highway agencyâs roadway, intersection, ramp, and crash data, but the software can provide substantial benefits by automating the diagnosis and countermeasure selection process. Economic analyses are not typically needed as part of crash history reviews. The identification of what the highway agency considers to be a potentially correctable crash pattern, especially a pattern involving severe crashes, is sufficient justification to include a design improvement in a 3R project. Cost-effectiveness analyses are typically applied where no crash patterns are found (see Section 3.3.3). However, some agencies may prefer to verify through economic analysis that correction of a specific pattern would be cost-effective. Both HSM Chapter 7 and Safety Analyst provide economic appraisal procedures that can be applied for this purpose. 3.3.2 Traffic Operational Analysis If the current traffic operational level of service for a roadway, or the anticipated LOS at any time during the service life of the planned pavement resurfacing (typically 7 to 12 years), is less than the highway agencyâs target LOS for the roadway, then a design treatment to improve the LOS is appropriate as part of a 3R project. Examples of design treatments that may improve LOS are lane widening, shoulder widening, and addition of turn lanes. It is also appropriate to consider alternatives to design improvements, such as changes in traffic control devices that might accomplish the same objective. Traffic operational analyses are generally performed with the procedures of the Highway Capacity Manual (HCM) (11). For any specific facility type, the HCM specifies the service measure(s) that define LOS (see Table 2), presents thresholds that define specific LOS categories, and present procedures to estimate the service measure(s) and, therefore, determine LOS. The HCM procedures provide one method to quantify the service measure(s), but other methods including field measurements and simulation modeling are also possible. Such alternatives to the HCM procedures can be applied at any site, but would normally be needed only for complex situations that fall outside the scope of the HCM procedures.
15 Table 2. Traffic Operational Service Measures for Specific Roadway Facility Types (11) Road Type HCM Service Measures Rural two-lane roadway Average travel speed Percent time-spent-following Rural multilane highway Traffic density Urban and suburban arterials Travel speed as a percentage of base free-flow speed Intersection design improvements may also be included to improve the LOS as part of a 3R project. Delay is a service measure used to define LOS at intersections. The addition of a left- or right-turn lane may make operational sense as well as provide safety benefits. The target LOS for any roadway is determined by the responsible highway agency. Design policies, such as the AASHTO Green Book or the policies of individual highway agencies, do not normally define target LOS criteria for application to specific projects. As a practical matter, most highway agencies find that appropriate target LOS levels vary with the project context, giving consideration to topography, area type (urban/rural), metropolitan area size, and project scope. Thus, the target LOS is often a project-by-project decision, and highway agencies have substantial flexibility in deciding whether to incorporate traffic operational improvements in particular 3R projects. 3.3.3 Cost-Effectiveness Analysis Cost-effectiveness analysis of 3R projects applies a systemic approach to safety assessment, based on crash prediction models, as opposed to the analysis of observed crashes discussed above for the crash history review in Section 3.3.1. Every roadway section has some nonzero risk of crash occurrence, even if no crashes have actually occurred. A systemic approach considers the need for design improvements based on potential crash risk, whether that risk has resulted in crashes or not. The cost-effectiveness analysis can be based solely on assessment of predicted crash frequency or predicted and observed crash frequencies can be applied using the Empirical Bayes (EB) procedure to estimate the long-term expected crash frequency. The type of cost-effectiveness analysis recommended for the assessment of 3R projects is benefit-cost analysis, in which the results can be expressed as benefit-cost ratios (project benefits divided by project costs) or net present benefits (project benefits minus project costs). The benefit-cost analysis procedures applicable to 3R projects are presented in Chapter 5. These procedures are tedious to apply manually, so two spreadsheet-based benefit-cost tools have been created for application by highway agencies; these benefit-cost tools are described in Chapter 5.6 of these guidelines. The benefit-cost tools can be applied by highway agencies in any of three ways: ï· evaluation of an individual candidate 3R project to determine whether a specific proposed design improvement would be cost-effective ï· evaluation of an individual candidate 3R project to determine whether any of a range of specific design improvements would be cost-effective
16 ï· evaluation of a set of potential improvements for generic 3R project scenarios to establish minimum AADT guidelines for consideration of specific design improvements for specific roadway types The first two applications listed above, involving analysis of individual candidate 3R projects, are the preferred approaches to benefit-cost analysis. Chapter 5 shows why these approaches are preferred. The third application listed above, establishment of minimum AADT guidelines for specific design improvements on specific roadway types, is less desirable than the first two applications, but is an acceptable approach for highway agencies that prefer not to conduct analyses of individual projects. Procedures for application of the three approaches listed above are presented in Chapter 5.