Quantifying the impact of road lighting on road safety – a New Zealand study
It is well known from the literature that road lighting has significant safety benefits. The NZTA Economic Evaluation Manual (EEM) quotes a 35% reduction in crashes as the effect of upgrading or improving lighting where lighting is poor. However, no well-established dose-response relationship to lighting parameters exists from which one can deduce benchmark levels of lighting for safety. This study looked at a sample of road lighting installations spread over the urban areas of nine territorial local authorities. Standard road lighting parameters were measured in the field using a variety of instruments including illuminance meter, luminance meter and digital camera. Field measurements were related to the ratio of night-time to day time crashes as a measure of night time safety vis-a-vis daytime safety. A statistically significant dose-response relationship was found between average road luminance and safety across all traffic volume groups, with an indication that the relationship may be stronger where more serious crashes are involved. Threshold increment was also a significant variable but not so longitudinal uniformity or overall uniformity. The results related to luminance will allow practitioners to better estimate the safety benefits of different levels of lighting resulting in better targeting of expenditure.