Evaluation of The Impact of Spectral Power Distribution on Driver Performance

CHAPTER 1. Introduction

Traditional roadway lighting uses high-pressure sodium (HPS) light sources, which provide high photometric efficacy. HPS light itself, however, is amber and does not render object color correctly. With the advent of light-emitting diode (LED) technology in roadway lighting, a new aspect of the light source is now being considered by roadway lighting designers and road agencies: its spectral power distribution (SPD). Broad-spectrum sources, with significant spectral output across the entire visible spectrum, potentially provide additional benefits to the driver because these light sources can provide better color information and can activate the photoreceptors in the eye more efficiently. These benefits from broad-spectrum sources may include the ability to reduce lighting levels without sacrificing visibility, improved visual performance (speed and accuracy of performing a visual task), better object color recognition, and increased visual comfort. A model has been developed that may predict these benefits, but it has not been verified in the field.⁽¹⁾ This project investigates these effects and considers the potential benefits of a broad-spectrum light source.

In addition to investigating the application of broad-spectrum sources to overhead lighting, this project highlights a significant issue in the design and analysis of the roadway visual environment. Lighting for motorists is typically provided by two different sources: headlamps and, if warranted and installed, a fixed overhead-lighting system. These two systems are typically independent. The design of a vehicle headlamp is specified by Society for Automotive Engineering (SAE) J1383-2010: Performance Requirements for Motor Vehicle Headlamps and Federal Motor Vehicle Safety Standard 108.^(3,4) Requirements for overhead lighting are documented in the American National Standards Institute–Illuminating Engineering Society of North America RP-8 publication, with more practical specifications in the American Association of State Highway and Transportation Officials Roadway Lighting Design Guide.^(5,6) Another interesting issue is that jurisdiction governing these two light sources is bifurcated: the National Highway Traffic Safety Administration regulates vehicle-based systems, while State and local agencies are responsible for roadway lighting infrastructure. Because these two systems are both designed and managed separately, their interaction is rarely considered in evaluating the visible road environment. Therefore, in addition examining the spectral effects of overhead lighting, the integration of vehicle headlamps and overhead lighting and the subsequent effect on visibility was also considered in this project.

The final aspect of the project was to take advantage of the potential spectral effectiveness of broad-spectrum lighting systems and of video detection and instrumentation systems to develop an integrated approach to highlight pedestrians on the side of the roadway. Because many of the spectral differences in the eye occur in the periphery of the visual field, a broad-spectrum peripheral detection system could potentially benefit visibility to the sides of the road. A momentary peripheral illumination (MPI) system was developed and tested for this project to determine whether this adaptive lighting system benefited drivers by increasing their detecting pedestrians and animals on and along the side of the roadway. The impacts of the MPI system on driver behavior were also investigated.

Project Objectives

The objectives of this project were to evaluate the following:

• Impact of the spectra of overhead-lighting systems on driver visual performance for higher speed roadways.
• Interaction of vehicle headlamps and overhead lighting in terms of object visibility.
• Applicability of mesopic models and scaling factors in a roadway lighting design.
• Impact of a peripheral illumination system on driver visual performance.

The approach for this project was multiphased. The first phase of the project involved a scoping experiment to identify the critical aspects of spectral interactions. The second phase evaluated the overall performance of the MPI system. Based on these two experiments, the subsequent phase of the project included a study of headlamp and overhead lighting interaction, and a study of the models of spectral impact of light sources. Last, an indepth study evaluated all of these factors in a single experiment.