Abstract
For over 50 years, the basis for calculating the length of sag vertical curves has been the sight distance based on a 1° upward divergence of the headlamp beam. Over the last 20 years, headlamps have transitioned from being 100% sealed beam to modern replaceable bulb headlamps that project less light above the horizontal plane compared to the sealed beam headlamps that are the basis for the sag curve design. This paper analyzes the performance of modern headlamps with respect to sag vertical curve design by calculating the amount of illumination reaching the pavement in a sag curve. The analysis indicates that the headlamp beam angle used to calculate sag curve length should change from 1 to 0.75° to provide an illuminance level with modern headlamps that is equal to the illuminance provided by sealed beam headlamps. Such a change would increase the K values for sag curve design from 13, 30, and 55 to 15, 37, and 70 for design speeds of 110, 80, and 50 km/h, respectively.
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