REEVALUATION OF THE USEFULNESS AND APPLICATION OF DECISION SIGHT DISTANCE

One of the most important elements of highway geometric design is sight distance. In "A Policy on Geometric Design of Highways and Streets", the American Association of State Highway and Transportation Officials has adopted a new sight distance standard known as decision sight distance (DSD). These sight distances are considerably longer than stopping sight distance, giving motorists additional margin for error and sufficient length to maneuver their vehicles at the same or reduced speed rather than to just stop. Nevertheless, there has been some concern that states have not adopted and implemented this standard. To determine if this is true, a limited survey of 15 states was made. A questionnaire was used to determine if the state has adopted the standard--and if it has not, why. Comments were solicited on how the standard should be modified. This paper also critiques a proposed revised AASHTO standard for DSD and concludes with the author's recommendation for a change to the DSD standard.