Decision sight distance (DSD) has been defined as the distance at which drivers can detect a hazard or signal in a cluttered roadway environment, recognise it or it's threat potential, select the appropriate speed and path, and perform the required action safely and efficiently. A research effort was devised and performed to relate this concept to specific road types, design speeds, traffic operating conditions, geometric features, and driver attributes. It was performed in two phases, with the following objectives: Phase I: Critically evaluate and synthesise relevant literature pertaining to DSD and derive values for highway design. Phase I led to the identification of a hazard avoidance process model as a basis for quantifying decision sight distance. The process includes three elements of information processing-detection recognition and decison-making followed by initial response and vehicle manoeuvre. The outcome of Phase I was the development of preliminary DSD values based on the estimated times for the various elements of the model as reported in the literature. Phase II: Validate, via highway field study, derived DSD values. In Phase II, 19 subjects drove an instrumented vehicle through eight typical highway situations in order to validate the preliminary values. In general, the results of the field study supported the derived DSD values, with some modifications, and confirmed that decision sight distance is operationally valid. Recommendations are presented on the decision sight distance criteria, and its application for design and traffic operations. (FHWA)
Abstract