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Publication Number:  FHWA-HRT-15-047     Date:  August 2015
Publication Number: FHWA-HRT-15-047
Date: August 2015


Evaluation of The Impact of Spectral Power Distribution on Driver Performance

CHAPTER 11. Application to Practice

The interpretation of these results is an important aspect of this project. As mentioned, the results of the experiments show that the impact of overhead lighting spectrum on driver visual performance is limited to specific situations. It is important to note that, in many situations, the broad-spectrum light source did not improve driver visual performance over the narrow-spectrum light source, but neither did it worsen driver visual performance. Other studies have shown benefits of the use of broad-spectrum light sources beyond providing better visual performance. In user preference studies, broad-spectrum light sources were preferred for their user comfort and acceptance.(23,101) Other research has shown that broad-spectrum sources provide for better object contrast, thus increasing the detection of objects along the roadside. These results indicate that broad-spectrum lighting is a valid choice in general and likely a desirable choice for roadway lighting.

LED lighting is the optimal broad-spectrum source for roadway lighting. The benefits of LED lighting, in addition to those of other broad-spectrum sources, are significant. Typically LED installations use half the energy of that used by traditional HPS roadway lighting. Similarly, LED lighting can be controlled and dimmed in adaptive-lighting designs that actively correlate lighting levels to environmental demands, saving even more energy. These are compelling reasons to use LED lighting, which does not reduce driver visual performance. Other factors such as circadian impacts, sky glow, and glare should be considered but were outside of the scope of this project.

The final aspect to consider regarding broad-spectrum lighting is the application of mesopic scaling factors. A significant finding of this research is the non-applicability of these scaling factors, based on the spectral component of the light source, to roadway lighting design criteria. It is important, however, to highlight that the results of this project are consistent with the Illuminating Engineering Society (IES) RP-8-14 Recommended Practice.(5) The IES position is that lighting levels can be scaled based on the light source spectrum if the lighting design is for a roadway with a posted speed limit of less than 40 km/h (25 mi/h). Scaling can be performed on roads in this category because the lighting system is primarily for the benefit of pedestrians, and vehicle headlamps provide adequate lighting for drivers. This project shows that there are no spectral effects of roadway lighting on driver visual performance. However, a pedestrian has a wider field of view, and there may be spectral effects of lighting on pedestrian visual performance.


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