N C H R P R E P O R T Evolutions in the Geometric Design of Highways and Streets Integrating Performance-Based Analysis B R I A N L . R AY H ighway and street geometric design has can be developed, assessed, and advanced based on The author is Senior evolved significantly in the past century , and their support of the desired outcomes for the project. Principal Engineer , the movement toward performance-based This can lead to customized solutions and can help Kittelson & Associates, approaches is reaching practitioners. Transportation guide project decision making. Advances in perfor- Inc., Portland, Oregon, design has incorporated performance-based mance-based methodologies support context-sensi- and Chair of the TRB approaches for many years, primarily in relation to tive and practical solutions to meet project needs Design Section. materials such as pavement or steel. Results from proj- and to maximize investments. ects sponsored by the National Cooperative Highway Sources of Standards Research Program (NCHRP) are bringing these approaches to highway and street geometric design. Early roadway design focused on the quality of travel Performance-based analysis allows professionals and on adapting to weather. Early in the history of to consider and recommend solutions that are more automobiles, low traffic volumes and relatively low effective and adaptable to the context of a project speeds made the quality of travel and year-round use than those based on compliance with a nominal the priorities. As traffic volume grew , and motorized dimensional value from a design standard or speci- vehicles became a dominant transportation mode fication. Geometric design solutions, for example, between 1920 and 1940, vehicle designs advanced, P HOTO : P IMPINELLUS , W IKIMEDIA C OMMONS TR NEWS 301 JANUARY–FEBRUARY 2016 The eastern span of the San Francisco–Oakland Bay Bridge was replaced in 2013 by a self-anchored suspension bridge that can withstand the largest earthquake expected in a 1,500-year period. Agen- cies are incorporating more performance-based approaches such as seis- mic design into highway infrastructure. 27
The “modern highway” on operational outcomes and safety performance. P HOTO : N EW Y ORK P UBLIC L IBRARY of the early 20th century Eventually, the Federal Highway Administration adapted quickly to (FHWA) established 13 roadway design criteria to automobile use; most provide operational uniformity and design consis- transportation policies tency , with the intent to attain desired safety perfor- focused on design mance. uniformity across jurisdictions. Need for Flexibility The evolution in roadway design has produced high- quality roadways serving a range of users and vehi- cle types. Applied research results have helped to speeds increased, and highway and street design quantify design criteria based on observed opera- practices evolved to react and to adapt. tions and safety performance. Nevertheless, despite Transportation policies emphasized design uni- advances in experience and software, the highway formity and consistency on similar roadway types and street design process has remained centered on between the states. This allowed consistent con- nominal design values or standards. Yet designers struction practices, materials, and a uniform experi- need to apply engineering judgment in their design ence for roadway users. The design of facilities was activities, as well as the flexibility inherent in pub- uniform and consistent, regardless of jurisdictions, lished design guidance. but the standards did not necessarily imply or con- Roadway agencies have limited financial re - sider a level of safety performance. sources and often develop projects within physical Standards evolved beyond consistency in dimen- constraints—such as a limited right-of-way in an sions and began to signify quality of performance in urban area or an area with specific environmental operation or safety. In the late 1960s and 1970s, sensitivities. Constructing roadways categorically to groups such as the Highway Research Board—prede- meet design standards, therefore, is not always fis- cessor to the Transportation Research Board—moved cally possible or reasonable. beyond materials testing to assess user needs and Through initiatives such as context-sensitive human factors and to establish design values focused solutions and performance-based practical design, NCHRP Publishes Performance-Based Framework for Roadway Design N CHRP Report 785, Performance-Based that best meet a project’s unique context. Analysis of Geometric Design of Highways The performance-based analysis framework in and Streets , documents a process framework NCHRP Report 785 helps practitioners develop for conducting performance-based analyses of solutions that highway geometric design. The methodology is based on understanding intended project u Facilitate walking, biking, and transit, in outcomes and then considering and selecting addition to serving passenger cars and goods the geometric design elements or features movement; TR NEWS 301 JANUARY–FEBRUARY 2016 Clear delineation of curves is another low-cost safety improvement. 28
professionals are able to apply flexible design Example low-cost safety treatments along a crash- approaches to construct roadways adapted to the prone stretch of roadway unique needs of each contextual environment. New may include guardrails approaches were needed to support contemporary and speed feedback planning and design decision making. A perfor- signs. mance-based approach could support project docu- mentation needs and inform and guide project decision making. Performance-Based Design Other technical areas have adopted performance- based approaches since the 1970s. For example, fire business- and safety-related decisions, seismic engi- safety design shifted from a code-compliance neering moved toward predictive methods for assess- approach to a systems approach—the focus was not ing potential seismic performance. on how thick a wall must be but on how much pro- Varying from Standards tection the wall could provide and for how long before burning through. Designers often must consider geometric solutions Similar changes in the evolution of seismic design with dimensions that differ from published values. led to changes in engineering practice and research Historically , engineers have considered design stan- in structural engineering. Engineers recognized that dards or other published nominal values as the mea- code-based strength and ductility requirements for sures for comparing and assessing design choices. designing new buildings were not always suitable With this approach, comparing design dimensions to for evaluating and upgrading existing structures. nominally accepted values often becomes a surrogate Applying performance-based engineering methods for relative safety performance. in seismic structural design meant that a building These decision-making approaches, however, must withstand a seismic event and minimize the cannot be used to document or support design loss of lives even if the structure becomes an eco- choices that require variances or to evaluate design nomic loss. With this emphasis on making rational exceptions. In some cases, a variant design choice NCHRP Report 785, Perfor - u Reduce crash frequency and severity; m ance-Based Analysis of u Enhance a community’s livability; Geometric Design of u Support economic development; and Highways and Streets , is u Support other context-sensitive and practi- available from the TRB online cal design considerations and approaches. bookstore, https://www.my trb.org/Store/Product.aspx? The performance-based approach supports ID=7394; to view the book online, go to http://www.trb. project documentation needs and can inform and org/Main/ Blurbs/171431.aspx. guide project decision making while supporting risk management. As NCHRP and AASHTO explore changes and approaches to improve the Transportation agencies have limited resources processes of highway geometric design, perfor- with many competing demands. Performance- mance-based analysis will play a central role. based analysis provides designers with new meth- TR NEWS 301 JANUARY–FEBRUARY 2016 A performance-based process framework con- ods and principles for customizing design siders the performance factors for particular geo- recommendations from a range of solutions metric design elements. Designers can consider appropriate to any design context or environ- and select design values or features based on the ment. The NCHRP Report 785 framework sup- impact that the resulting geometric design per- ports a range of initiatives, including context- formance has on the intended project outcomes. sensitive design and solutions, performance- NCHRP Report 785 documents ways to consider based practical design, flexibility in design, and apply published design criteria for roadways complete streets, and multimodal design. This and to assess a design’s performance in terms of frame work represents a fundamental, positive accessibility, mobility, quality of service, reliability, advance in the evolution of highway and street and safety. geometric design. 29
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