Lighting Levels for Isolated Intersections: Leading to Safety Improvements

Abstract

A number of studies have linked the benefits of roadway lighting to a reduction in crash rates at night for a variety of roadway types including rural intersections. Assessing the effectiveness of intersection lighting has primarily relied upon crash database modeling comparing lighted intersections to unlighted intersections. The current research effort gathered similar metrics for comparison but also measured the amount of lighting within isolated rural intersections. Sixty-three intersection locations were chosen for lighting measurement from six different counties within Minnesota. A vehicle mounted illuminance meter data collection system was used to collect data at each intersection. The data collection system utilized five separate illuminance meters and captured horizontal illuminance while driving through all 63 intersection locations. Following data collection, a series of negative binomial regression models were used to assess the horizontal lighting level in conjunction with the nighttime crash ratio, intersection configuration type, and proximity of an intersection to a curve in the roadway. The first model used data from the lighted an unlighted intersections. The results showed that across all intersections, an increase in the average horizontal illuminance (3.91 lux) by 1-lux (~0.09 fc) reduced nighttime crash rates by 9%. A second model used only lighted intersection data and showed an increase in 1-lux from average (6.41 lux) reduced crashes by 20%. A third and final model used unlighted intersections only. A 1-lux increase from average (0.20 lux) or increasing illuminance to lighted levels (as defined by the modeling), reduced nighttime crash ratios by 94%. Intersection configuration and proximity to curves also affected the nighttime crash ratios. The results of this effort are discussed in terms the impact of horizontal illuminance on crash rates. Additional items discussed include thresholds for minimum and maximum lighting levels and future research investigating and validating these efforts.