For more than 50 years, headlight sight distance has been one method for calculating the length of sag vertical curves; the others are passenger comfort, drainage control, and general appearance. The headlight sight distance is based on a 1 upward divergence of the headlamp beam. Over the past 20 years, headlamps have transitioned from being 100% sealed beam to modern replaceable bulb headlamps that project less light above the horizontal plane than the sealed-beam headlamps that are the basis for sag curve design. This paper describes theoretical and field measurements of the illuminance levels provided by sealed-beam and modern headlamps. The results of the analysis indicate that modern headlamps provide significantly less light above the horizontal than sealed-beam headlamps, indicating a potential need to modify the design equations for sag vertical curves. According to the theoretical analysis, the upward divergent headlamp angle used in the sag curve design equation should be reduced from 1 to between 0.75 and 0.90. The field analysis indicated a significant difference in illuminance levels from the theoretical analysis, but the results also indicate a need to reduce the headlamp angle used in sag curve design.