Grade impact investigation during stopping sight distance determination on 3D road environment

In the present research, Stopping Sight Distance (SSD) adequacy is assessed through a three-dimensional approach, which associates road visibility in terms of grade effect. SSD adequacy is controlled through the difference between the available and the required SSD. The research is focused on a right turned, two lane rural road, associated with a crest vertical curve for a given speed value. The road design is in accordance with the Greek Road Design Guidelines (OMOE-X, 2001) by utilizing the control design parameters for a design speed of 80km/h and a wide range of grade values, where the vertex point of the crest vertical curve is positioned at fixed points along the road axis. The investigation is based on a vehicle speed exceeding the design speed by 20km/h. In total 1874 cases of compound alignments were examined and the results revealed that the available sight distance is decreasing while grade increases. The vehicle speed on the tangent sections was proved unacceptable in terms of providing SSD adequacy. However, it was found that SSD adequacy is granted when the vertical vertex falls inside the circular arc of the curve’s horizontal alignment, while the optimum area is defined shortly before the horizontal vertex point.