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53Â Â This is the first empirical study to quantify melatonin and alertness responses to the intensity of and spectrum of electrical roadway lighting in healthy drivers. The results of the current study can be summarized as follows: â¢ One aspect of driver sleep health, as measured by salivary melatonin suppression, is not significantly affected by LED roadway lighting even at light levels higher than those specified in IES RP-8-18. â¢ There are no differences in salivary melatonin suppression between LED and HPS roadway lighting when measured at the same light level (roadway luminance of 1.5Â cd/m2 or a corneal illuminance of 1.9Â lux). â¢ An increase in the LED light level did not increase salivary melatonin suppression. â¢ The only statistically significant differences in salivary melatonin suppression were observed between positive control and the roadway lighting conditions. Light levels in the positive control (3500Â lux) were considerably higher than those experienced in the roadway lighting conditions even at the higher level (1.9Â lux). â¢ Driver alertness as measured objectively by detection and color recognition distances (reaction times), PERCLOS, and SDLP did not show significant differences between the HPS, LED, and no-roadway-lighting conditions. â¢ Major detection and color recognition distances were dependent on the color of the object and the light level. An increase in the light level increased the detection distances. â¢ Detection and color recognition distances for the HPS roadway lighting were also affected by exposure time, where an increase in the exposure time reduced the detection and color recognition distances. â¢ Subjective driver alertness as measured by KSS did not show significant differences between the HPS, LED, and no-roadway-lighting conditions. The only statistical differences observed were between the positive control and the rest of the roadway lighting conditions. â¢ Illuminance dosages from the roadway lighting conditions (from both HPS and LED sources) are considerably lower than the illuminance dosages experienced from consumer electronic devices such as televisions and tablets. â¢ At the light levels specified in IES RP-8-18, HPS and LED roadway lighting did not affect driversâ salivary melatonin suppression, objective alertness, or subjective alertness between 1:00 AM and 3:00 AM. â¢ The results of the present study, although limited in scope, do not support the need to modify the guidelines for LED roadway lighting. It should be noted, however, that roadway lighting (LED and HPS) does have a detrimental effect on sky glow, light pollution, flora, and fauna. Adequate care should be taken to minimize these impacts. C H A P T E R Â 6 Conclusions