Stopping sight distance: can we see where we now stand?

TLDR: Conclusions are drawn regarding the appropriateness of the current methodology and several specific recommendations are offered for additional research on this important topic.

Abstract: This paper examines the development of stopping sight distance (SSD) methodology over the past 75 years. Publications between 1914 and 1940 show that sight distance became increasingly important, but that it was not thoroughly understood. The emphasis during this period was on letting drivers see other vehicles in sufficient time to take evasive action. This concept changed drastically with the 1940 publication of AASHO's classic methodology, which made specific reference to objects, eye heights, and driver perception and reaction times. Evidence shows that the new procedures were gradually assimilated into the design process. Since 1940, emphasis has been on fine tuning the methodology by modifying its parameters. The paper discusses the prominent factors affecting SSD and traces their development over the past 75 years. The sensitivity of stopping sight distance to changes in the key parameters is examined. Characteristics and weaknesses of the methodology are discussed through a review of the recent technical literature. Five potential problems with the current AASHTO policies are discussed. Conclusions are drawn regarding the appropriateness of the current methodology and several specific recommendations are offered for additional research on this important topic.