A new stopping sight distance model for use in highway geometric design

TLDR: This model is based on parameters describing driver and vehicle capabilities that can be validated with field data and defended as safe driving behavior and results in stopping sight distances, sag vertical curve lengths, and lateral clearances that are between the current minimum and desirable requirements and crest vertical curves that are shorter than current minimum requirements.

Abstract: Stopping sight distance is an important design parameter in that it defines the minimum sight distance that must be provided at all points along the highway. Thus, it influences geometric design values, construction costs, and highway safety. Stopping sight distance is defined as the sum of two components - brake reaction distance and the braking distance. The basic model for calculating stopping sight distances was formalized in 1940, and the model's parameters have been altered to compensate for changes in eye height, object height, and driver behavior over the past 50 years. Recent studies, however, question whether the model's parameters and assumptions represent real-world conditions. This paper presents a new model for determining stopping sight distance requirements for geometric design of highways. The new model is based on parameters describing driver and vehicle capabilities that can be validated with field data and defended as safe driving behavior. More than 50 drivers, 3,000 braking maneuvers, 1,000 driver eye heights, and 1,000 accident narratives were used in developing the recommended parameter values for the new model. The recommended values are attainable by most drivers, vehicles, and roadways. The new model results in stopping sight distances, sag vertical curve lengths, and lateral clearances that are between the current minimum and desirable requirements, and crest vertical curve lengths that are shorter than current minimum requirements.