A Performance-Based Highway Geometric Design Process (2016)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

3 The design of a highway—its three-dimensional features (horizontal alignment, vertical alignment, and cross section) and appurtenances to provide for drainage, traffic control, and safety—requires a well-defined process. AASHTO and its predecessor, AASHO, have published highway design policy since the 1940s; the underlying highway design process has remained essentially unchanged since that time. That process has the following characteristics: • Dimensionally based, with design values for physical dimensions directly derived from tables, charts, and equations. • Requires establishment of fundamental design controls including location, terrain, and functional classification. • Requires designers to make choices for other major factors (e.g., design speed, design hour volume, design vehicle) that will influence subsequent design decisions from within established ranges. • Based on selection of a design speed, and in some cases design vehicular traffic volume, other design criteria are directly derived or obtained for minimum dimensions (e.g., lane width, curve radius) and/or maximum dimensions (e.g., grade) as appropriate for the design controls and assumptions. • Direct performance measures in terms of vehicle mobility, including speed and level of service (LOS), are explicitly considered in some design decisions (e.g., number of lanes). • Costs versus benefits are also an integral part of the design process, but are implicitly considered through recommended dimensional ranges for different area and terrain types. • Nominal safety is presumed through the application of the process and technical guidance, but safety performance may not be explicitly considered. • Relies on mathematical models as the basis for derivation of dimensional values (e.g., point-mass model for selection of curve radius and superelevation). NCHRP Project 15-25, “Alternatives to Design Speed for Selection of Roadway Design Criteria,” examined a critical step in the traditional design process. The January 2005 Interim Report for that project points out that “While design speed has always been central to the geometric design process, actual road designs are strongly influenced by design speed in some cases, but not in all cases.” That report also notes that “on portions of the highway removed from sight obstructions and horizontal curves, highways designed for a broad range of design speeds may look nearly identical.” That project explored alternatives to that portion of the traditional process but lacked the necessary resources and scope to develop a comprehensive solution. During the past 60 years, transportation needs have changed and much has been learned about the relationships among geometric design, vehicle fleet, human factors, safety, and operations. AASHTO has continually updated its policies to respond to these changes, but such updates have provided limited changes to the fundamental process or basic design approaches. Some agencies C h a p t e r 1 Introduction

4 a performance-Based highway Geometric Design process have begun using an expanded array of roadway functional classifications as a basis for selecting certain design criteria. An assessment of the current design process is needed to ensure that recent advances in knowledge [e.g., the AASHTO Highway Safety Manual (HSM)] and emerging issues (e.g., complete streets, flexible design) are appropriately addressed. The objective of this research is to develop a comprehensive, flexible design process to meet the needs of geometric designers in the future. The process considers: • Specification of the project purpose and need, including the modes that will be using the facility. • Context setting of the facility. • Desired performance outcomes for the facility for the various modes; including safety, mobility, and access management. • Methods for evaluating trade-offs associated with different design alternatives. • Optimization of the design given the project’s financial and other constraints. • Flexibility to address issues that arise from stakeholder involvement or environmental reviews. • Documentation of decisions to address tort liability concerns.