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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-04-132
Date: December 2005

Enhanced Night Visibility, Volume I: Executive Summary

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Glare studies were conducted in Phase II and Phase III of this project. In Phase II, a discomfort glare study evaluated 11 different headlamp configurations by rating the driver's discomfort glare caused by oncoming UV–A VESs as compared to other VESs (ENV Volume VII). To more fully understand the effects of glare on vehicle safety, a disability glare evaluation in combination with a discomfort glare evaluation was conducted as part of Phase III (ENV Volume XV). In addition to discomfort glare ratings, this study measured driver detection distances of pedestrians in the glare of various oncoming HID and halogen headlamps.


The primary focus of the Phase II discomfort glare study (ENV Volume VII) was to determine the degree of driver discomfort caused by oncoming supplemental UV–A headlamps. The study included all of the UV–A configurations from the visual performance studies and the two baseline headlamps types (HLB and HID) assessed alone. Three additional headlamp-only systems were also included for a total of 11 VES configurations. The discomfort glare study was conducted on the Smart Road using 60 participants split equally among three age groups: younger participants (18 to 25 years), middle-aged participants (40 to 50 years), and older participants (60 years or older). Participants drove at 40 km/h (25 mi/h) toward a fixed glare source (i.e., each VES) and rated it twice using the deBoer discomfort rating scale.(6) The first rating represented the discomfort the participant experienced from a range of approximately 396 to 305 m (1,300 to 1,000 ft) away from the opposing headlamps. The second rating reflected the discomfort experienced in the range of approximately 137 to 46 m (450 to 150 ft).

The hybrid UV–A headlamps appeared to have added discomfort glare relative to the baseline headlamps (recall that hybrid UV–A headlamps had a significant visible light component), but the other UV–A headlamps did not. The halogen headlamps selected for this testing produced more discomfort glare than did the high intensity discharge headlamps tested. This result may have been caused by the aiming strategy used for the halogen headlamps; however, a subsequent study could not confirm that the aiming strategy made a difference. Analysis of illuminance measurements taken at the approximate driver's eye position during testing indicated that the amount of visible light (i.e., maximum illumination) directed toward the driver's eye by the opposing headlamps was the overriding factor contributing to the reported discomfort sensation; the spectral distribution of the headlamp light source did not appear to have an effect.


The purpose of the discomfort and disability glare study (ENV Volume XV) was to determine the effect of beam intensity and pattern on disability glare, defined here as detriment to pedestrian detection, and to determine any relationship between disability glare and discomfort glare.

The discomfort and disability glare study used the baseline headlamps from the Phase II discomfort glare study and three additional HID headlamps. These five VESs allowed comparison of different combinations of beam intensities and patterns:

  • High/narrow: higher intensity with narrow beam pattern (HID).

  • High/wide: higher intensity with wide beam pattern (HID).

  • Low/wide: lower intensity with wide beam pattern (baseline HID).

  • Medium/medium: mid-level intensity with medium-width beam pattern (HID).

  • Low/narrow: low intensity with narrow beam pattern (baseline HLB).

To study the effect of environmental light on discomfort and disability glare, a dashboard light source mounted in the experimental vehicles produced two driver light adaptation levels, 0.15 lux (lx) and 0.45 lx at the driver's eye level. The disability glare study used two pedestrians in white clothing as objects, differing only in their locations relative to their glare sources; both pedestrians stood 15.2 m (50 ft) behind their glare source, one pedestrian on the centerline and the other on the right edgeline. Thirty participants were again divided equally into three age groups-younger (18 to 25 years), middle-aged (40 to 50 years), and older (65 years or older).

During the study, the participants drove toward the glare sources, which were positioned on a static frame simulating an oncoming vehicle. During the disability glare assessment portion, participants indicated when they could detect the pedestrian as they approached the glare sources. During the discomfort glare portion, participants were asked to rate the discomfort experienced (using the deBoer scale) over an approximately 305-m (1,000-ft) approach toward the glare source.

Three dependent variables were collected during this study: pedestrian detection distance, the deBoer scale rating of discomfort glare, and driver illumination level (i.e., illuminance at the driver's approximate eye position.)

The results indicated that beam intensity (i.e., maximum light output) affected disability and discomfort glare more than beam pattern did. Specifically, VESs with higher maximum output had shorter pedestrian detection distances and were rated as more discomforting. In general, the results showed that discomfort glare corresponded to disability glare; oncoming VESs that were rated as more discomforting were the same VESs that restricted detection distances.


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