Improving Safety at Signalized Intersections near Vertical Curves

The research described herein documents an investigation into countermeasures that can be used to provide motorists with advance notification of traffic signals or queues from those signals that are located beyond the motorists' line of sight due to a crest vertical curve. 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 a vertical curve and a traffic signal beyond the curve where SSD is provided but DSD is not. Essentially, motorists within the RDZ are provided with SSD for unexpected stopping but are not provided with the added decisionmaking 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. In cases where a traffic signal or queue from a signal is located within the RDZ for an existing roadway, guidance is provided on both the type and location of countermeasures that can be used. Results of field tests conducted using both the suggested type and location of advance warning sign countermeasures are included within this report and its appendix.