Sight distance red zones on combined horizontal and sag vertical curves

Coordination of highway horizontal and vertical alignments is based on subj ective guidelines in current standards. This paper presents a quantitative analysis of coordinating horizontal and sag vertical curves that are design ed using two-dimensional standards. The locations where a horizontal curve should not be positioned relative to a sag vertical curve (called red zones ) are identified. In the red zone, the available sight distance (computed u sing three-dimensional models) is less than the required sight distance. Tw o types of red zones, based on stopping sight distance (SSD) and preview si ght distance (PVSD), are examined. The SSD red zone corresponds to the loca tions where an overlap between a horizontal curve and a sag vertical curve should be avoided because the three-dimensional sight distance will be less than the required SSD. The PVSD red zone corresponds to the locations wher e a horizontal curve should not start because drivers will not be able to p erceive it and safely react to it. The SSD red zones exist for practical hi ghway alignment parameters, and therefore designers should check the alignm ents for potential SSD red zones. The range of SSD red zones was found to d epend on the different alignment parameters, especially the superelevation rate. On the other hand, the results showed that the PVSD red zones exist o nly for large values of the required PVSD, and therefore this type of red z ones is not critical. This paper should be of particular interest to the hi ghway designers and professionals concerned with highway safety.