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vi AUTHOR ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 15-41 by the Center for Infrastructure Based Safety Systems (CIBSS) at the Virginia Tech Transportation Institute (VTTI). Dr. Ronald B. Gibbons, Director of the CIBSS, was the Project Director and Principal Investigator. The other authors of this report are Alejandra Medina, Senior Research Associate with VTTI and co-Principal Investigator; Dr. Hesham A. Rakha, P.Eng, Director of the Center for Sustainable Mobility at VTTI and Professor with the College of Civil & Environmental Engineering at Virginia Tech, and co-Principal Investigator, Brian Williams, Research Specialist Senior with CIBSS; and Jianhe Du, Research Associate with the Center for Sustainable Mobility at VTTI. The work was done under the general supervision of Dr. Gibbons. The research team would like to thank Mr. Jonathan Stover from the Virginia Department of Transportation for his review of the project plan and the results.
vii ABSTRACT This document is the final report for NCHRP Project 15-41, âSag Vertical Curve Design Criteria for Headlight Sight Distance.â This report includes a review of the current methodologies used in the design of sag vertical curves, a review of the changes in headlamp technologies, the results of the survey of practitioners, two visibility experiments, and discussion on potential changes to the American Association of State Highway and Transportation Officials (AASHTO) design guide. The review of the headlamp technology shows that, over time, headlamp technologies have had increasing limitation on the amount of light emitted above the horizontal axis of the headlamp. In addition to the regulatory impact, headlamp technologies such as visually optically aligned technologies also limit uplight. In the practitioner survey, it was found that very few deviations from the AASHTO design methodologies were used. Based on the practitioner review, the potential to modify the current methodologies is limited to the manipulation of the vehicle speed, deceleration, and the angle of curvature change. The results of the visibility experiments found that participants detected objects at distances which were significantly shorter than the safe stopping distance (SSD). This occurred not only in sag vertical curves, but also on flat roadway. This indicated that even if sag vertical curves were redesigned, visibility distance would still be shorter than SSD because the headlamps would be the limiting factor. A review of the potential modifications to sag vertical curve designs (which were suggested as a result of the practitioner survey) found that these changes would be inadequate to make up the difference between visibility distance and SSD.
1 CHAPTER 1 INTRODUCTION The evolution of the typical vehicle headlamp has undergone a significant transition in recent years. Traditional headlamps were simply sealed-beam lamps, whereas modern headlamps are well-designed optical instruments that provide more effective light control than was previously attainable. The transition of the headlamp to these new modern styles coupled with the new height of headlamps in larger profile vehicles has resulted in some significant changes to vehicle performance in the driving environment. The first change relates to general visibility. New headlamp designs have shown limited effects in pedestrian and object visibility (Blanco et al., 2005) and variability in sign visibility (Carlson and Hawkins, 2003). The most recent considerations that have been investigated are those of sag vertical curve design. Gogula (2006) and Hawkins (2007) have performed extensive evaluations of the performance of modern headlamps in sag vertical curves. In their research, they have determined that the current design criteria of sag vertical curves are not representative of existing technology. As a vehicle approaches a sag vertical curve, the distance that the headlamps reach is limited by the road that rises on the other side of the curve relative to the vehicleâs central axis. The greater the change in the gradient from one side of the curve to the other, and the shorter the overall curve distance, the more limited the headlamp performance. If a headlamp projects a greater amount of light above its horizontal axis, the limitations that impact the headlamp are minimized. The AASHTO design requirements for sag vertical curves are based on four specifications: headlamp sight distance (SD), passenger comfort, drainage control, and general appearance. The headlamp SD requirements are based on a safe stopping SD that is perceived throughout the sag curve. The assumption used in the development of the requirements is that a 1-degree uplight limitation is evident in the headlamp. However, recent research on sag vertical curves and headlamps show that this limitation is likely no longer valid and that real-world performance from a modern headlamp does not meet the current design standards. These results have been developed in a theoretical manner and Gogula (2006) has attempted to valid them. RESEARCH OBJECTIVE This project considers the evaluation of the performance of modern headlamps in vertical sag curve conditions. There are three primary research objectives: 1) Evaluation of the impact of modern headlamp performance on the design of sag vertical curves; 2) Development of proposed changes to the criteria and guidance for the design of sag vertical curves based on the results of the visibility testing; and 3) If required, perform a cost analysis of the potential changes to the AASHTO policy. To achieve these goals, the project team has undertaken a series of tasks which led to the final assessment of the AASHTO methods for the design of sag headlamps. The tasks described in this document include the literature review, a survey of practitioners, an assessment of the current methodologies, and consideration of headlamp performance.
