Study Details

Study Title: Safety Effects of Street Illuminance on Roadway Segments in Florida

Authors: Wang et al.

Publication Date:JAN, 2017

Abstract: Nighttime crashes are over-represented on the highway system in the United States. Roadway lighting, providing additional visibility by supplementing vehicle headlights, has been identified as an effective countermeasure to improve nighttime safety. However, the effects of street lighting illuminance parameters in reducing nighttime crashes on roadway segments are not well-documented. This study aimed to investigate the effects of street lighting illuminance, rather than the presence of street lighting, on nighttime crash occurrence on roadway segments. Illuminance data were collected from 403 roadway segments in the Tampa Bay area in Florida from 2012–2014 using the Advanced Lighting Measurement System matched with four years of crash data (2011–2014). Zero-inflated negative binomial models were developed based on the collected data for addressing the issues of over-dispersion and excessive zero observations in the sample. The major conclusions include the following: (1) mean horizontal illuminance significantly and near-logarithmically reduces nighttime crash frequency and night-to-day crash ratio; (2) good illuminance uniformity (max/min < 6) can significantly reduce expected nighttime crash occurrence by 24.6% (based on expected nighttime frequency) or 2.3% (based on night-to-day ratio), compared to poor uniformity (max/min ≥ 6); (3) compared to the night-to-day crash ratio, the expected nighttime crash frequency-based analysis tends to overestimate the safety performance of average illuminance; and (4) considering that the night-to-day crash ratio can hedge the influence from the confounding or interacting variables that are related to illuminance, the CMF based on the night-to-day crash ratio is preferred.

Study Citation: Wang, Z., P. Lin, Y. Chen, P. Hsu, S. Ozkul, and M. Bato. "Safety Effects of Street Illiuminance on Roadway Segments in Florida". Presented at the 96th Annual Meeting of the Transportation Research Board, Paper No. 17-05438, Washington, D.C., (2017).

Comments: The Clearinghouse includes only the CMF and CMFunction that used the night-day (N-D) ratio to account for confounding factors, given that the authors seemed to imply that this was a more reliable finding.


CMFs Associated With This Study

Category: Highway lighting

Countermeasure: Change average illuminance from 1 to X

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
CMF EquationCRF Equation4 StarsDay time,NighttimeAllNot specified

Countermeasure: Improve street lighting illuminance uniformity

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.9772.34 StarsNighttimeAllNot specified