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3 S E C T I O N 1 1.1 Background The FHWA has, since 1985, designated 13 specific design elements as controlling criteria for roadway design (1, 2). These 13 controlling criteria are the following: ⢠Design speed ⢠Lane width ⢠Shoulder width ⢠Bridge width ⢠Structural capacity ⢠Horizontal alignment ⢠Vertical alignment ⢠Grade ⢠Stopping sight distance ⢠Cross slope ⢠Superelevation ⢠Vertical clearance ⢠Horizontal clearance Highway construction and reconstruction projects on the National Highway System (NHS) and resurfacing, restora- tion, and rehabilitation (RRR) projects on NHS freeways must meet the established design criteria for the design ele- ments listed above, or a formal design exception must be pre- pared and approved. Highway agencies are also encouraged to follow these criteria for non-NHS projects (2). The estab- lished design criteria for geometric elements on construction and reconstruction projects are those in either the 2001 or 2004 editions of the AASHTO Policy on Geometric Design of Highways and Streets (3, 4), commonly known as the Green Book. The differences between the 2001 and 2004 editions of the Green Book are minor except in the area of superelevation. The 2011 edition of the Green Book (5) has been published by AASHTO, but has not yet been adopted by FHWA in a for- mal rulemaking for application to the NHS. Different design Introduction procedures and criteria apply to RRR projects (6), but the 13 controlling criteria are still key elements in the design of such projects. The Green Book presents numerous geometric design ele- ments and dimensional criteria. With the intent of focusing attention on the most important criteria and the understand- ing that a design exception evaluation for every design ele- ment would be impractical, FHWA identified the 13 design criteria listed above as having substantial importance to oper- ational and safety performance (1, 2, 7). Although all excep- tions from applicable design policies should be documented in some manner, FHWA has established the 13 controlling criteria as requiring formal design exceptions for the types of projects noted above. But, are the 13 controlling criteria actually the most impor- tant design criteria for safety and efficiency? Moreover, is the current design exception process for these controlling criteria effective in achieving the most efficient investment of limited resources to improve safety and operations? Substantial new knowledge about safety and operations has been gained since the 13 controlling criteria were established in 1985. Further- more, an industry-wide movement away from standards-based design to a more flexible process in which each design is tai- lored to fit into the context of community and environmen- tal values has been under way since the 1990s (8). Software tools such as SafetyAnalyst (9) and the Interactive Highway Safety Design Model (IHSDM) (10) have been created to ensure explicit consideration of operations and safety and cost-effective investment of resources within a more flexible design process. Ultimately, this could lead to a performance- based design process in which the traffic operational and safety effects of design decisions are considered explicitly. The AASHTO Guidelines for Geometric Design of Very Low- Volume Local Roads (ADT ⤠400) (11) has also created a flexi- ble approach to design appropriate for these very low-volume facilities.
4Research is needed to address key questions concerning the 13 controlling criteria and their application in current practice including the following: ⢠What has been learned about the relationship between the controlling criteria and safety and operations? ⢠Based on current knowledge, are highway agencies using the appropriate controlling criteria? Should some criteria be dropped or combined? Should others be added? ⢠Are each of the controlling criteria applicable to all road- way types or should the controlling criteria vary by roadway type? Answers to these questions are needed to ensure that the design process (including the process for design exceptions) focuses on the design elements that have the most substantial safety and operational impacts. This research provides tech- nical information that could lead to more flexible approaches to design of highway projects, where appropriate. The timing of the research is very appropriate given the publication of the first edition of the AASHTO Highway Safety Manual (HSM) (12) and an updated edition of the Highway Capacity Manual (HCM) (13) in 2010. Much of the recent discussion on design flexibility has focused on safety issues because the traffic opera- tional effects of design features are reasonably well established, but knowledge about their safety effects has been growing rap- idly. For this reason, the following discussion emphasizes safety effects, but the research reported here definitely included full consideration of both operational and safety effects. In previous years, when the safety effects of specific design criteria assumed to be important to safety were unknown, requiring compliance with those criteria, except in limited cases, was the most rational approach to providing appropri- ate levels of crash frequency and severity on a new or recon- structed facility. This approach is referred to in the HSM as achieving nominal safety. However, since the typical or aver- age effects on crash frequency and severity of many geometric design criteria are now documented in the HSM (and others have been quantified in this research), a new paradigm based on quantitative analysis and/or cost-effectiveness analysis has become feasible. This new paradigm is referred in the HSM as substantive safety. Such a process could ensure that, where roadways are upgraded to meet established design criteria, the incremental funds spent in the name of safety actually produce a safety benefit commensurate with their cost. However, an assessment is clearly needed as to whether the state of safety knowledge has reached a point that can fully support such a process. Such an assessment has been performed in this research. This research has addressed how far toward revised control- ling criteria and a revised design exception process the trans- portation industry can move based on current knowledge and new knowledge obtained during the research. 1.2 Research Objectives and Scope The objective of this research was to describe the impact of the controlling roadway design criteria on safety and opera- tions for various urban and rural roadway types. The research has considered how the current controlling criteria or possible modified criteria (adding, dropping, or combining particu- lar design elements), through the design exception process, influence the provision of flexibility of the design process. The scope of the research was limited to roadway design cri- teria and, based on the research scope established by NCHRP Project 17-53, has not addressed intersection design, roadside design, or access control. The project scope includes new con- struction and reconstruction projects, but not RRR projects. This research has established priorities among the 13 con- trolling criteria and suggested directions in which they might evolve, but no specific policy recommendations for changing the 13 controlling criteria have been made. The research has considered how the 13 controlling criteria could be appropri- ately applied in non-federal projects. 1.3 Organization of NCHRP Report 783 The remainder of NCHRP Report 783 is organized as fol- lows. Section 2 reviews each of the 13 controlling criteria for design, their traffic operational and safety effects, and appro- priate mitigation strategies when the controlling criteria can- not be met. Section 3 reviews the design exception practices of highway agencies in using the 13 controlling criteria. Sec- tion 4 presents the results of new research, providing expanded knowledge on the traffic operational and safety effects of the 13 controlling criteria. Section 5 suggests potential refinements to the criteria definitions. Section 6 presents priorities for the 13 controlling criteria. Section 7 discusses the interpretation of the research results. Section 8 presents the conclusions and recommendations of the research.
