Incorporating safety in the design of a highway is one of the foremost duties of a designengineer. Design guidelines provide standards that help engineers include safety in thedesign of various geometric features. However, design guidelines are not frequentlyrevised and do not accommodate for the frequent changes in vehicle design. One suchexample is the change in vehicle headlamps. These changes significantly impact theilluminance provided on the road and in turn the design formula.Roadway visibility is critical for nighttime driving. In the absence of roadwaylighting, vehicle headlamps illuminate the road ahead of a vehicle. Sag vertical curvedesign depends on the available headlight sight distance provided by the 1 degreeupward diverging headlamp beam. The sag curve design formulas were developed inthe early 1940s when sealed beam headlamps were predominant. However, headlampshave changed significantly and modern headlamps project less light above the horizontalaxis. In this research, the difference in illuminance provided by sealed beam headlampsand modern headlamps was examined. For the theoretical analysis, three different sagcurves were analyzed. On these curves, about 26 percent reduction in illuminance was observed at a distance equal to the stopping sight distance when comparing sealed beamto modern headlamps. A change in the headlamp divergence angle from 1.0 degree to0.85 degree will provide the required illuminance on the road when using modernheadlamps. A field study was performed to validate the theoretical calculations. It wasobserved that for modern headlamps, a divergence angle less than 1 degree and greaterthan 0.5 degrees will provide illuminance values comparable to sealed beam headlamps.As a part of this research, a preliminary study, examining the impact of degradedheadlamp lenses on the illuminance provided on sag vertical curves was conducted. Asignificant reduction in illuminance reaching the roadway on sag curves was observed,due to headlamp lens degradation.
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