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Performance-Based Analysis of Geometric Design of Highways and Streets (2014)

Chapter: Chapter 2 - Geometric Design Decision Making and Performance

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Suggested Citation:"Chapter 2 - Geometric Design Decision Making and Performance." National Academies of Sciences, Engineering, and Medicine. 2014. Performance-Based Analysis of Geometric Design of Highways and Streets. Washington, DC: The National Academies Press. doi: 10.17226/22285.
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Suggested Citation:"Chapter 2 - Geometric Design Decision Making and Performance." National Academies of Sciences, Engineering, and Medicine. 2014. Performance-Based Analysis of Geometric Design of Highways and Streets. Washington, DC: The National Academies Press. doi: 10.17226/22285.
×
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Suggested Citation:"Chapter 2 - Geometric Design Decision Making and Performance." National Academies of Sciences, Engineering, and Medicine. 2014. Performance-Based Analysis of Geometric Design of Highways and Streets. Washington, DC: The National Academies Press. doi: 10.17226/22285.
×
Page 10
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Suggested Citation:"Chapter 2 - Geometric Design Decision Making and Performance." National Academies of Sciences, Engineering, and Medicine. 2014. Performance-Based Analysis of Geometric Design of Highways and Streets. Washington, DC: The National Academies Press. doi: 10.17226/22285.
×
Page 11
Page 12
Suggested Citation:"Chapter 2 - Geometric Design Decision Making and Performance." National Academies of Sciences, Engineering, and Medicine. 2014. Performance-Based Analysis of Geometric Design of Highways and Streets. Washington, DC: The National Academies Press. doi: 10.17226/22285.
×
Page 12
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Suggested Citation:"Chapter 2 - Geometric Design Decision Making and Performance." National Academies of Sciences, Engineering, and Medicine. 2014. Performance-Based Analysis of Geometric Design of Highways and Streets. Washington, DC: The National Academies Press. doi: 10.17226/22285.
×
Page 13
Page 14
Suggested Citation:"Chapter 2 - Geometric Design Decision Making and Performance." National Academies of Sciences, Engineering, and Medicine. 2014. Performance-Based Analysis of Geometric Design of Highways and Streets. Washington, DC: The National Academies Press. doi: 10.17226/22285.
×
Page 14
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Suggested Citation:"Chapter 2 - Geometric Design Decision Making and Performance." National Academies of Sciences, Engineering, and Medicine. 2014. Performance-Based Analysis of Geometric Design of Highways and Streets. Washington, DC: The National Academies Press. doi: 10.17226/22285.
×
Page 15

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8C H A P T E R 2 This chapter provides an overview of geometric design decisions within the project devel- opment process and the relationship between project-level and geometric design performance measures. In addition, the geometric design activities within each project stage, environmental evaluations, and context-sensitive design approaches are discussed. 2.1 Overview of Geometric Design Decision Making This section outlines the various activities of the project development process and the role of and relationship between geometric design activities within the various project development stages. Geometric design has limited roles in system planning; performance considerations and outcomes of geometric design decisions become most relevant during the alternatives identi- fication and evaluation and preliminary design stages. Beyond that point, as more key design decisions are made, there is less flexibility to make significant performance-based decisions. Discrete design choices become increasingly finite through final design as plans, specifications, and estimates are prepared. Projects are commonly identified via actions associated with plan- ning activities. Catalysts could include categories such as safety, operations, economic develop- ment, land development, capital improvement, maintenance, or other initiators. Many of these catalysts could be the same as intended project outcomes. Systems planning may include rudimentary considerations of geometric design in the broad- est terms of classifying the roadway facilities (i.e., a rural, multilane, limited-access facility with interchanges or grade separations at major roadways and minor cross streets). As projects are advanced from planning, there may be some consideration of the intended outcomes such as an improvement in safety, an increase in localized segment or node capacity, or general upgrading of roadway corridor elements. In all cases, there are broad ideas of the nature and magnitude of the project, and the impending project development activities help define, refine, and select solutions within the unique context of each project. This document is focused on geometric design decisions and their performance effects. Under- standing where a geometric design activity fits within the project development process and how geometric design decisions and activities influence or are influenced by other activities within the project development process may guide geometric design decision making. Section 2.3 provides descriptions of the project development stages used within this document. For the purposes of this report, the project development stages are defined as planning, alternatives identification and evaluation, preliminary design, final design, and construction. The diagram shown in Exhibit 2-1 highlights some of these general relationships. The exhibit is simplified to show general relationships and is not intended as an absolute. For example, Geometric Design Decision Making and Performance

Geometric Design Decision Making and Performance 9 meaningful stakeholder engagement can begin in the planning stages and continue through construction. Similarly, a designer may investigate roadway profiles or bridge type constructa- bility while identifying and evaluating alternatives even if the overall geometrics are at the 15% design level. 2.2 Relationship between Project and Geometric Design Performance Within the project development activities, the project’s overall effectiveness can be estimated or predicted and evaluated through construction when the facility is open. Once constructed, the actual project performance can be observed. Performance evaluations of the permanent geometric design elements (i.e., not the construction work zone design elements) generally peak in the middle stages of preliminary design (15% to 30%). Beyond 30% plans, the design choices and performance measures become increasingly discrete as the plans, specifications, and estimates are completed. Project-level outcomes can relate catalysts such as safety performance targets, congestion relief, or better service for multimodal users. However, project drivers may also include elements such as “livability,” “community cohesion,” or “economic development.” In some project con- texts, attributes of livability may include maintaining a rural character, making the area “walk- able,” or preserving an area’s history or culture. Community cohesion might include strong land use connections and relationships, network connectivity to support various users’ mobil- ity, or having projects with a minimal footprint. The perceived effectiveness of a project will be influenced by geometrics and their corresponding performance. For example, the choice of signalization or curb and gutter could be perceived as diminishing the rural character of an area. Or, increasing vehicle or bicycle capacity by removing parking in a commercial/business area may be seen as counter to economic development. Section 3.2, Project Performance, discusses project performance goals and measures. This section also highlights USDOT strategic goals The perceived effectiveness of a project will be influenced by geometrics and their corresponding performance. Exhibit 2-1. Geometric design decisions within the project development process.

10 Performance-Based Analysis of Geometric Design of Highways and Streets along with performance target categories from MAP-21. Geometric design choices and their resulting performance can directly influence and be influenced by project performance goals, objectives, or targets. Exhibit 2-2 conceptually depicts the level at which performance outcomes of geometric design decisions are central to decision making throughout the project development process. The exhibit shows geometric design performance is less of a consideration in the early plan- ning stages when so many project issues are being considered. In the alternatives development stage, geometric design decisions and their outcomes become central to project discussions and considerations. As alternatives and concepts are screened and others refined and advanced to more detailed evaluations, other project considerations may become more of a focus. As final design plans are completed, the role of geometric decisions diminishes. As a project advances to construction, other project issues may be central to decision making. Measures related to overall project context may be identified early and conceptually in plan- ning stages. They are a critical element in helping inform and guide the range of alternatives. Each identified alternative’s general evaluation and corresponding performance measures are closely connected to the geometric design elements and their individual and collective perfor- mance. These stages create some of the highest amount of interaction between the geometric design outcomes and the intended project outcomes as the concepts are refined and advanced. Performance measurements for the geometric design elements become more refined. During preliminary design, more design details and evaluations are performed on a decreasing number of alternatives. Ultimately, a single alternative is selected and advanced to final design. Project decisions are documented and the selected alternative is developed to a level of detail to support construction. When the project advances to final design, the geometric design measures become increasingly discrete, as needed, to finalize the design details. Project context performance measures are then focused on quantifying context-sensitive impacts during construction. This could include topics such as preserving access during construction, Exhibit 2-2. Geometric design performance measures within the project development process.

Geometric Design Decision Making and Performance 11 defining the number of lanes that will remain open at any given time, or the quality of service expected for the range of work zone users. As outlined in NCHRP Report 581: Design of Construc- tion Work Zones on High-Speed Highways (1), there could be some geometric design decisions associated with constructing temporary roadways or configurations. For the purposes of this discussion, those are not included in this report. 2.3 Geometric Design and the Project Development Stages For the purposes of this report, the project development process is defined as consisting of the following five stages. Federal, state, and local agencies may have different names or other nomenclature, with the objective being to advance from planning to implementation. As shown in Exhibit 2-2, overall project objectives and performance measures are a primary consideration. Geometric design performance measures are considered at a lower level of detail. 2.3.1 Planning Studies Planning studies are not explicitly included in this report. However, planning could include limited geometric concepts of the general type or magnitude of project solutions to support programming. 2.3.2 Alternatives Identification and Evaluation The project needs identified in prior planning studies inform concept identification, develop- ment, and evaluation. Geometric design decisions and geometric design performance become paramount considerations at this stage. Understanding the project context and intended out- comes allows potential solutions to be tailored to meet project needs within the opportunities and constraints of a given effort. FHWA describes context-sensitive solutions as “a collaborative, interdisciplinary approach that involves all stakeholders in providing a transportation facility that fits its setting” (2). In considering the concept of “context-sensitive design/solutions,” this stage continues the meaningful and continuous stakeholder engagement to be carried through- out the project development process. This stage establishes and documents intended project outcomes that will influence and be influenced by geometric design decisions. Design elements may be developed to a 15% design level, and it is possible a single alternative could be selected at this stage. It is not uncommon for multiple alternatives to be advanced to preliminary design for additional review and evaluation before identifying a preferred alternative. The overall elements that often occur in this project development stage include the following (3): • Project initiation • Purpose and need • Traffic analyses • Preliminary alternatives • Public outreach • Technical studies • Cost/benefit evaluations • Refined analyses • Selected alternative(s)

12 Performance-Based Analysis of Geometric Design of Highways and Streets If needed, state or federal environmental review and impact documentation efforts begin in this stage. A discussion of the general environmental review and impact documentation activi- ties is included in Section 2.4, which highlights where and how geometric design and project performance in the alternatives identification and evaluation and preliminary design stages sup- port the environmental review and impact documentation activities. 2.3.3 Preliminary Design Concepts advancing from the previous stage are further refined and screened during preliminary design. In more complex, detailed, or high-impact projects, the preliminary design (30% plans) and subsequent documentation is used to support more complex state or federal environmental clearance activities. The corresponding increased geometric design detail allows refined technical evaluations and analyses that inform environmental clearance activities. Preliminary design builds upon evaluations conducted as part of the previous stage (alternatives identification and evalua- tion). Some of the common components of preliminary design include the following (3): • Horizontal and vertical alignment design • Typical sections • Grading plans • Structures • Traffic/intelligent transportation systems • Signing and striping • Illumination • Utilities The expected performance effects of geometric design influence project outcomes and, ulti- mately, inform project decision making. As design concepts advance from concept to 30% design, iterations and revisions help hone the design, and the performance effects of geometric design decisions have a relatively significant influence on project decisions. As the designs advance from 30% to 100%, there are relatively few significant geometric changes. Based on the proposed performance-based model depicted in Exhibit 1-1, during these itera- tions the concepts are refined as needed to best achieve the intended outcomes. These could be broader project outcomes (e.g., speed management or attaining a certain level of mobility) or design specific (e.g., providing an acceptable horizontal alignment while avoiding an environ- mentally sensitive property). 2.3.4 Final Design The design elements are advanced and refined in the final design stage. Typical review peri- ods include 60%, 90%, and 100% plans before completing the final set of plans, specifications, and estimates. During this stage there is relatively little variation in design decisions as the plan advances to 100%. Functionally, in this stage of the project development process, the targeted performance measures have a lesser degree of influence on the form of the project. This relation- ship was presented in Exhibit 2-2. 2.3.5 Construction Construction activities are not explicitly included in this report. Geometric design decisions may be related to temporary roadways, connections, or conditions that facilitate construction. Project performance measures may relate to project context elements and could guide or inform temporary construction decisions within the intended project outcomes and within the completed project.

Geometric Design Decision Making and Performance 13 2.4 Geometric Design and Environmental Evaluations and Clearance This section summarizes how geometric design and performance-based decisions relate to state and federal environmental policy act considerations. Even without environmental clear- ance needs, the early stages of project development strive to understand project scope and develop alternatives responding to a project-specific purpose and need. If an environmental review and documentation effort is needed, performance-based evaluations of geometric design can support environmental activities. This section is intended to help establish where and how environmental review processes may influence or be influenced by performance-based analysis of geometric design of highways and streets. Whether state or federally mandated, environmen- tal evaluations typically occur in the early stages of the project development process, often being completed at the early stages of preliminary design. Preliminary design intended outcomes, and measured or projected performance, are often documented in technical reports supporting associated environmental documentation requirements. The following are the general elements of an environmental evaluation process: • Project scoping • Purpose and need • Alternatives analysis • Affected environment • Environmental consequences • Mitigation Exhibit 2-3 depicts where environmental clearance often occurs. The encircled areas high- light the relationship between the level of environmental clearance and the project development Exhibit 2-3. Geometric design and environmental clearance during project development.

14 Performance-Based Analysis of Geometric Design of Highways and Streets stage. The circle on the left reflects relatively low levels of environmental clearance needs such as a categorical exclusion in the National Environmental Policy Act (NEPA). As described by FHWA, “NEPA established a national environmental policy intentionally focused on federal activities and the desire for a sustainable environment balanced with other essential needs of present and future generations of Americans” (4). State-level environmental evaluation com- monly includes similar checklist-type documentation efforts. The circle on the right reflects projects that might be more complex or extensive or have proj- ect sensitivities. These complex projects often require more extensive design details (up to 30% design) to provide the engineering and technical evaluations and documentation to support appli- cable environmental reviews and clearances (such as a finding of no significant impact or record of decision in NEPA environmental assessments and environmental impact statements, respectively). If a project has an environmental review component, existing conditions and no-build analy- ses help define the scope and magnitude of the range of possible solutions. Project goals and objectives help define the purpose of, and need for, the solutions. Early stakeholder engagement helps define the intended project outcomes and performance measures. In some cases, these may be project context measures related to elements outside the geometric design performance outcomes (such as having a walkable community, preserving a rural character, or creating a high level of traffic capacity). Section 3.2.2 describes some of these types of goal topics. Stakeholder engagement and the resulting project “scope” will influence fundamental design choices during the alternatives identification and evaluation stages of the project development process. In summary, environmental review and documentation efforts for relatively low-impact projects can range from checklists and categorical exclusions to more complex environmen- tal processes such as environmental assessments or environmental reports. More complex or higher-impact projects may require the highest level of environmental review and documen- tation such as an environmental impact statement in NEPA. As the degree of environmental review complexity increases, it is common for corresponding engineering evaluations to become more detailed because preliminary design stage efforts guide the technical support and studies. In this case, completed preliminary design in conjunction with environmental clearances allows a project to advance to final design. Throughout the environmental review efforts, performance-based analysis of geometric design elements can inform and support project decision making. 2.5 Context-Sensitive and Flexible Design Approaches Several published documents, such as AASHTO’s A Guide for Achieving Flexibility in Highway Design (5), discuss the role of context-sensitive approaches to considering project solutions that are adapted to the local planning and design context. “Flexible” designs may be used to adapt to a local context and to achieve intended project outcomes. Sections 3.1.1 and 3.1.2 present fundamental questions for understanding project context elements and considering intended project outcomes. Context-sensitive and flexible design approaches can stem from considering “Whom are we serving?” and “What are we trying to achieve?” By understanding the various users to be served and the overall project outcomes, applicable geometric solutions can be explored. Considering geometric performance of potential solutions can inform project decision making since intended project outcomes may be influenced by the geometric design considerations. FHWA and AASHTO have emphasized “flexibility in highway design” as a means to help estab- lish context-sensitive solutions. In addition, FHWA and AASHTO emphasize the importance of continuous and meaningful stakeholder engagement to help establish each project’s context

Geometric Design Decision Making and Performance 15 and identify a range of possible solutions applicable to each project environment. Performance- based analyses of geometric design elements provide the means to support flexible design solu- tions or elements to adapt to unique project needs. Early stakeholder engagement can influence and be informed by geometric design considerations. The concept of flexible design and the degree of centrality of the geometric design in the early project development stages is reflected in Exhibit 2-2. As projects evolve from preliminary to final design, the design choices and influ- ences become increasingly finite. Documenting design decisions and the considerations supporting those choices that result in flexible design solutions is a key component in managing tort liability. Having a process frame- work for understanding intended project outcomes, and a logical means to consider a range of design choices and solutions, provides a reproducible and objective methodology. Adapting a project to a context may lead to design variances or exceptions. Flexible design approaches may lead to geometric values or configurations outside published design values. FHWA’s Interactive Highway Safety Design Model (IHSDM) is a suite of software analy- sis tools used to evaluate the safety and operational effects of geometric design decisions on highways (6). IHSDM is a decision-support tool. It provides estimates of a highway design’s expected safety and operational performance and checks existing or proposed highway designs against relevant design policy values. Results of the IHSDM support project decision making by summarizing the geometric performance elements of alternative geometric design elements or configurations. The AASHTO HSM (7) provides factual information and proven analysis tools for crash frequency prediction. The HSM helps users integrate quantitative crash frequency and severity performance measures into roadway planning, design, operations, and maintenance decisions. HSM analytical tools allow users to assess the safety impacts of transportation project and pro- gram decisions. These tools support context-sensitive or flexible design decision making. Performance-based tools such as IHSDM and HSM applications can support and inform design decision making for projects of any context. And documenting the evaluation methods and factors leading to geometric design values contributing to flexible design configurations can support a comprehensive risk management strategy. A repeatable process for performance- based analysis of geometric design can help manage risk in general, and support design decisions and documentation content for design variances and exceptions. 2.6 References 1. Mahoney, K. M., R. J. Porter, D. R. Taylor, B. T. Kulakowski, and G. L. Ullman. NCHRP Report 581: Design of Construction Work Zones on High-Speed Highways. Washington, D.C.: Transportation Research Board of the National Academies, 2007. 2. Federal Highway Administration (FHWA) and American Association of State Highway and Transportation Officials (AASHTO). Results of Joint AASHTO/FHWA Context-Sensitive Solutions Strategic Planning Pro- cess, Summary Report. Raleigh, N.C.: Center for Transportation and the Environment, North Carolina State University, 2007. 3. Van Schalkwyk, I., E. A. Wemple, and T. R. Neuman. Integrating the HSM into the Highway Project Develop- ment Process, FHWA-SA-11-50. Washington, D.C.: Federal Highway Administration. 4. Federal Highway Administration. Environmental Review Toolkit (http://www.environment.fhwa.dot.gov/ projdev/index.asp). Washington, D.C. 5. American Association of State Highway and Transportation Officials. A Guide for Achieving Flexibility in Highway Design. Washington, D.C.: 2004. 6. Federal Highway Administration. Interactive Highway Safety and Design Model. Washington, D.C.: 2003. 7. American Association of State Highway and Transportation Officials. Highway Safety Manual. Washington, D.C.: 2010. Performance-based analyses of geomet- ric design elements provide the means to support flexible design solutions or elements to adapt to unique project needs.

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 785: Performance-Based Analysis of Geometric Design of Highways and Streets presents an approach for understanding the desired outcomes of a project, selecting performance measures that align with those outcomes, evaluating the impact of alternative geometric design decisions on those performance measures, and arriving at solutions that achieve the overall desired project outcomes.

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