A Performance-Based Highway Geometric Design Process

Overview

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).