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GUIDELINES FOR USING DECISION SIGHT DISTANCE AT SIGNALIZED INTERSECTIONS NEAR VERTICAL CURVES

Whereas standard roadway design ensures that stopping sight distance (SSD) is provided at all locations along a roadway, there is no standard established for when decision sight distance (DSD) is needed with respect to traffic signals. A reduced decision zone (RDZ) was identified in the research as the location along a roadway with vertical curve and a traffic signal beyond the curve where SSD is provided but DSD is not. Essentially, motorists with the RDZ are provided with SSD for unexpected stopping but are not provided with the added decision-making and response time that DSD might otherwise provide as they approach the vertical curve and the downstream traffic signal. Contained within this report are techniques for determining whether an RDZ exists along an existing roadway or has the potential to exist in a proposed design. It is suggested that intersections not be located within the RDZ.