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Suggested Citation:"Chapter 1 - Introduction." 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 1 - Introduction." 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 1 - Introduction." 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 1 - Introduction." 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 1 - Introduction." 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 1 - Introduction." 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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2C H A P T E R 1 This section provides a foundation for the subsequent process framework by describing the role and value of performance-based activities in geometric design of highways and streets. It provides the guiding principles of this report while outlining the fundamental model of the performance-based approach. The chapter closes with an overview of the concepts of overall project and geometric design performance. These basic concepts are central to the process framework. 1.1 Role of Performance-Based Analysis in Transportation Activities Gone are the days of large publicly funded projects where funding magnitude was a primary consideration. Federal and state dollars were available as long as a state or local match could be generated. Public transportation funds are typically restricted to maintaining the integrity of the existing highway system and providing focused improvements for safety and/or operations within the current built-out system. Public works projects of all scales are more sensitive to fund- ing than ever before. And in many cases, cost magnitude and cost effectiveness play increasingly large roles in scoping projects. Often, reconstruction projects are limited in scope or available funding, or may be affected by physical constraints or social or environmental considerations. In some locations, especially constrained locations, designing to “full standards” simply is not feasible, with design variances, deviations, or exceptions becoming commonplace. Adaptive and flexible designs customized to each project context become increasingly preferred to make the most of project investments. Public-private partnerships are becoming more prevalent as transportation agencies consider new or retrofitted corridors serving managed lanes or freight facilities. Design-build contracting methods have been well established, resulting in significant design process differences compared to historical design-bid-build contracts. Financial catalysts and return-on-investment needs add a new dimension to low-cost and efficient designs. As cost-effective solutions of public-private partnerships and modified contracting vehicles become more prevalent, engineers and planners will remain responsible for making the most of project investments. Regardless of a project’s origin, performance-based analysis of geometric design provides a principles-focused approach that looks at the outcomes of design decisions as the primary mea- sure of design effectiveness. As public agencies meet transportation needs with less funding or engage in partnerships to support locally generated (sometimes development-funded) projects, the ability to make informed design decisions will likely increasingly rely on performance-based analysis results. Performance-based analysis of geometric design provides a principles-focused approach that looks at the outcomes of design decisions as the primary measure of design effectiveness. Introduction

Introduction 3 1.2 Role and Value of Geometric Design of Highways and Streets The Federal Highway Administration (FHWA) publication Flexibility in Highway Design (1) and American Association of State Highway and Transportation Officials (AASHTO) pub- lication A Guide for Achieving Flexibility in Highway Design (2) emphasize the importance of applying “flexibility” as documented in the recent editions of AASHTO’s A Policy on Geometric Design of Highways and Streets (3). Flexibility in geometric design has been supported for years, and increasingly in recent years, tools like the Interactive Highway Safety and Design Model (4) and publications such as the Highway Safety Manual (HSM) (5) and FHWA’s Speed Concepts: Informational Guide (6) provide the means to consider and measure geometric design perfor- mance. There is an increasing realization within the design community, supported by the tort liability and risk management community, that simply designing to standards does not reduce a professional’s risk for being sued. In addition, designing to standards does not always achieve an optimum design. Performance-based analyses are an integral part of project design documenta- tion, providing a foundation for tracking and supporting design decisions. A solid documenta- tion regimen supported by performance-based analyses can support flexible geometric design decisions. This flexibility allows designers to implement solutions in financially or physically constrained environments and makes project design decisions informed by anticipated geomet- ric design performance. 1.3 Guiding Principles of the Approach The following principles will guide users in creating usable, practical, and long-lasting high- ways and streets: • Intended outcomes: Fundamentally, the intent is to document the importance of and need for establishing each project’s “intended outcomes” and then focusing on performance-based analysis of geometric design to assess if intended outcomes have been achieved. In some cases, general project intended outcomes may influence geometric design elements and targeted per- formance. In other cases, geometric design performance may influence general project out- comes. During any of the project development stages, varying degrees of performance analyses guide discrete design decisions. • Connection to project development process: Users benefit from considering the project development process and the discrete activities (such as environmental evaluation and doc- umentation). This considers the opportunities to apply performance-based analysis to the geometric design of highways and streets, where and how the range of flexibility to influence project or design outcomes varies within each project development stage, and the general availability of data needed to support performance-based analysis at different project devel- opment stages. • Performance measures of design decisions: The primary focus is the performance effects of geometric design decisions. In some cases, other intended outcomes of the project may influence geometric design decisions; in other cases, the resulting effects of geometric deci- sions may influence or support broader project outcomes. This document summarizes and prioritizes specific measures that are sensitive to geometric design decisions within the cat- egories of access and accessibility, mobility, reliability, safety, and quality of services. These categories are consistent with broader, national performance-based transportation decision- making efforts (such as with those in the Moving Ahead for Progress in the 21st Century Act (MAP-21), described in Section 3.2).

4 Performance-Based Analysis of Geometric Design of Highways and Streets 1.4 Fundamental Model of the Approach Exhibit 1-1 illustrates the following basic steps in performance-based analysis to inform geo- metric design: 1. Identify intended project outcomes (desired project performance). This may include any number of project context-driven categories that help to identify a project need or pur- pose. Chapter 3 summarizes USDOT strategic goals, including “economic competitiveness,” “livable communities,” “safety,” and “state of good repair.” The 2012 surface transportation bill (MAP-21) identified performance categories, including “congestion reduction,” “envi- ronmental sustainability,” “freight movement,” and “system reliability.” Community residents and stakeholders may use terms such as “livability,” “community cohesion,” and “economic development.” Regardless of the nature of the source, these project outcomes (or project performance) help establish the measures by which project and geometric design performance might be measured. 2. Establish geometric design decisions. This could include establishing design criteria and developing preliminary designs. Whether the project is as discrete as finding ways to chan- nelize a right-turn lane to improve pedestrian crossing times or as broad as conducting an urban freeway corridor study, design options are considered by way of a variety of geometric design decisions. Chapter 2 discusses geometric design decisions and their changing emphasis through the various stages of the project development process. 3. Evaluate the performance of the geometric design. This is the point at which the perfor- mance outcomes of the geometric design choices are evaluated. Whether this is the general footprint of an interchange or the computed speed of the right-turn lane of a roundabout, establishing the geometric performance allows an assessment of the effectiveness of the design decision in relation to intended project outcomes. Chapter 4 presents information on assess- ing geometric design decisions and performance. Chapter 5 presents a performance-based analysis application framework. Chapter 6 provides six project examples and applications of the Chapter 5 framework considering the content of the balance of the report. Exhibit 1-1. Fundamental model for performance-based analysis of geometric design of highways and streets.

Introduction 5 4. Iterate design and outcomes to optimize. Depending upon the results of the assessment of geometric design performance in relation to intended project outcomes, there can be an iterative process to refine geometric design decisions to bring resulting performance in line with intended project outcomes. If an acceptable solution is not attainable, it may be neces- sary to re-evaluate intended outcomes. For example, if the original intended outcome was to provide congestion relief between two roadways and all interchange forms have unacceptable impacts, it may be necessary to reconsider intended project outcomes and establish a range of potential solutions offering congestion relief at some lower-than-originally-desired target performance. 5. Evaluate benefit/costs. In this step, the benefits and associated design choices are assessed to establish the value of the geometric solution compared to the intended project outcomes. If two concept solutions may meet project objectives and all other considerations are equal, the one providing the greater value would likely be advanced. 6. Select or advance project(s) or alternatives. As project alternatives are deemed viable within the project context, they may be advanced for more detailed evaluations and/or environmental reviews. Chapter 2 describes some typical relationships between alterna- tives evaluations and environmental review considerations in relationship to the project development stages. In summary, once specific issues to be addressed have been clearly articulated, identifying a proj- ect’s intended outcomes (project performance) as the basis for evaluating performance results is the first step in performance-based evaluations. In some cases, the project may be a well-defined and focused technical exercise to enhance a segment or node geometrics (for example, consid- ering options to increase intersection sight distance). In other cases, the project may include a variety of intended outcomes where selected solutions vary with desired project performance (for example, addressing traffic capacity needs in a multimodal, sensitive way through an his- toric downtown main street corridor). With intended outcomes defined (project or geometric), users may assess the performance results of alternative geometric design values or configurations to optimize potential project solutions within each project’s contextual design environment. 1.5 Overall Project and Geometric Design Performance Overall project performance may influence and may be influenced by geometric design decisions and their resultant performance. Mea- suring the effectiveness of overall project performance depends on the goal, intended outcome, nature, or catalyst for the project. Is a safety project initiated to address a documented crash severity or frequency issue? Are certain users overrepresented in crashes? If so, overall proj- ect performance might be measured by the expected change in crash frequency or severity, or by the expected change for certain users. Clearly, geometric design choices or geometric design alternatives will influence the outcomes. However, the ultimate measure of project success may not hinge upon the specific geometric element or value of a specific treatment, solution, or mitigation. For example, a single-lane roundabout may be a geometric solution with better expected safety performance than a signal- ized intersection in a given location. However, if the footprint of the roundabout precludes its application, the signalized intersection with protected left-turning movements may be the most appropriate geometric configuration for the project conditions. And even though the signalized intersection may not offer the theoretical safety performance benefits of the roundabout, its application could lead to a “successful” project outcome compared to the existing and forecast no-build scenario. An overall project performance goal may be to reduce crash frequency and severity. Geometric design performance goals may be to reduce conflict points and vehicle speeds.

6 Performance-Based Analysis of Geometric Design of Highways and Streets Project performance can include other elements that may not be specific to common trans- portation outcomes of capacity, safety performance, or quality of service for multimodal users. Project performance could include other aspects such as implementing a highway, street, or design element within a specified project budget or construction timeline. The perceived success of the project may not rely on any specific design element; however, the design elements or choices may, in fact, influence the project performance. Consider two intersection alternative configura- tions. One option might require right-of-way or result in expensive utility impacts compared to another configuration. Or one alternative could impact sensitive lands (wetlands or park land), requiring additional time to attain local, state, or federal permitting approval. In these two exam- ples, the choice of the geometrics could influence the cost and implementation schedule that was a measure of success for overall project performance. In some cases, an acceptable project outcome may be simply achieving an acceptable geomet- ric solution. For example, a local community wishing to support living-wage jobs may welcome a new manufacturing plant requiring a new interchange on a state highway. The project area may be constrained or the spacing between adjacent interchanges may be less than desirable. The potential employer may have some defined monetary contribution or investment for the improvement, above which it is not economically or financially feasible for the employer to establish operations in that location. In this case, project sponsors and state transportation pro- viders may be incentivized to find creative solutions to develop a financially feasible interchange that allows new access and supports the desired land uses. The overall success of the project may be to obtain the new access and do so in a way that adapts to the constrained environment, while being implemented within a limited budget and project time frame. Geometric design performance may be measured by the ability to achieve acceptable (not ideal) traffic operations, geometric design, safety performance, and signing and marking. Performance-based analysis can help guide project decision making. Geometric performance can greatly influence whether a project achieves intended outcomes. Specific design choices will result in operating speeds, operating environment, driver expecta- tions, and safety performance. Depending on the intended project outcomes, the results of geo- metric design decisions (geometric design performance) may or may not meet overall project needs. Consider a community where “Main Street” is a state highway. A local community may be striving for a walkable community with reduced travel speeds that promote adjacent develop- ment or facilitate comfortable and safe street crossings. A desired overall project performance measure may be to retain the local community culture and character or to improve economic vitality by changing the traffic volumes and patterns on Main Street. The choices made by the designer can directly influence the community’s character and the transition into and out of that community. Designing gateway features or cross-section changes at the highway transitions to Main Street can influence the tone and character for approaching drivers. As drivers leave Main Street to continue on the highway, the transition design elements can help maintain an environ- ment or operational quality established through Main Street. The choice of on-street parking, curb radii, and lane widths may influence speeds, crossing distances, or characteristics in the community. The choice of roundabouts at the community edge will directly influence travel speeds and predictive safety performance. In this example, the choice of geometric design ele- ments will yield explicit operational and safety characteristics for each user. The overall project performance may be directly linked to the specific design choices—and the specific performance of the design alternatives considered. In summary, performance-based analysis of geometric design provides a principles-focused approach that looks at the outcomes of design decisions as the primary measure of design effec- tiveness. Identifying project intended outcomes (project performance) as the basis for evaluating performance results is the first step in performance-based evaluations. Geometric performance

Introduction 7 can greatly influence whether a project achieves intended outcomes. Specific design choices greatly influence operating speeds, operating environment, driver expectations, and safety per- formance. Depending on intended project outcomes, the results of geometric design decisions (geometric design performance) may or may not meet overall project needs. As professionals address transportation needs in various project contexts, performance-based analysis results will support informed decision making. 1.6 References 1. Federal Highway Administration. Flexibility in Highway Design. Washington, D.C.: 1997. 2. American Association of State Highway and Transportation Officials. A Guide for Achieving Flexibility in Highway Design. Washington, D.C.: 2004. 3. American Association of State Highway and Transportation Officials. A Policy on Geometric Design of High- ways and Streets. Washington, D.C.: 2011. 4. Federal Highway Administration. Interactive Highway Safety and Design Model. Washington, D.C.: 2003. 5. American Association of State Highway and Transportation Officials. Highway Safety Manual. Washington, D.C.: 2010. 6. Federal Highway Administration. Speed Concepts: Informational Guide. Washington, D.C.: 2009.

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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.

This project has also produced a supplemental research materials report and a PowerPoint presentation.

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