Christopher J Edwards

Relationship of Vertical Illuminance to Pedestrian Visibility in Crosswalks

Twenty-six participants evaluated a series of crosswalk lighting designs by visually detecting objects at each crosswalk location while traveling in a moving vehicle. The research was performed on a closed test track under nighttime conditions while the participants were driving an SUV with regular halogen headlamps. The conditions included several vertical illuminance levels (6, 10, 20, and 30 lux), varied luminaire types [high-pressure sodium (HPS) and metal halide (MH)], and various target object types (pedestrian and surrogate objects). Only one age group of participants (66 years and older) was used for the study, with equal representation of males and females. The participants were asked to detect objects at each crosswalk location when they were confident an object was present. The results indicated that object detection distances changed on the basis of vertical illuminance level, luminaire type, and object type. Object detection distance for HPS was greatest at 30 vertical lux and for MH at 20 vertical lux. However, these results were moderated by the clothing color of the target object. When object color was considered, pedestrians in white clothing were identified earlier under the HPS lighting condition at 20 lux. Under the MH configuration, denim-clothed objects were detected earlier than black-clothed objects, especially at the 20-lux lighting level. The results suggest that a vertical illuminance level of 20 lux at crosswalk locations provides adequate levels for target object detection. In addition to benefiting from vertical illuminance, target objects that wore white clothing had detection distances superior to other object types of different clothing colors. Recommendations for crosswalk lighting configurations are further discussed.

Relationship Between Roadway Illuminance Level and Nighttime Rural Intersection Safety

Almost all existing research about intersection lighting indicates that the presence of lighting reduces nighttime crashes. This study aimed to quantify the effect of lighting level and lighting quality on the night-to-day (ND) crash ratios at rural intersections. Illuminance measurements were collected from 99 lighted and unlighted rural intersections in Virginia. The measurements were then combined with the crash data, obtained from the Virginia Department of Transportation, for each of the intersections. A negative binomial regression was used to model the crash and lighting data. Results indicated that lighting level (average horizontal illuminance) significantly affected the ND crash ratios at rural intersections. A 1 lux (lx) increase in the average horizontal illuminance at all rural intersections in Virginia corresponded to a 7% decrease in the ND crash ratio. For the lighted intersections, a 1 lx increase in average horizontal illuminance corresponded to a 9% decrease in the ND crash ratio. The largest decrease in the ND crash ratio was for unlighted intersections, where a 1 lx increase in the average horizontal illuminance corresponded to a 21% decrease. Stop-controlled intersections had smaller ND crash ratios than signalized intersections. Intersections with a posted speed limit of less than or equal to 40 mph had lower ND crash ratios than intersections with a posted speed limit of greater than 40 mph. Each of the results is discussed with respect to the impacts of the lighting measurements on nighttime driving safety and overall visibility.