Evaluation of Impact of Modern Headlamp Technology on Design Criteria for Sag Vertical Curves

Abstract

This paper 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 a survey of practitioners and two visibility experiments, and conclusions. The review of the headlamp technology shows that, over time, headlamp technologies have increasingly limited 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 systems also limit the upward divergence of the light beam from the longitudinal axis of the vehicle (uplight). The practitioner survey found that very few deviations from AASHTO design methodologies were used. On the basis of the practitioner review, the potential to modify current methodologies is limited to the manipulation of the vehicle speed, deceleration, and change in angle of curvature. The results of the visibility experiments showed that participants detected objects at distances that were significantly shorter than the stopping sight distance (SSD), not only in sag vertical curves but also on flat roadway. This finding 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 the design of sag vertical curves (which were suggested in the responses to the practitioner survey) found that these changes would be inadequate to make up the difference between visibility distance and SSD.