A new visibility criteria for roadway lighting

The ratio of the nighttime accident rate to the daytime accident rate per million miles is the best non-subjective measure of the effectiveness of fixed roadway lighting. Small Target Visibility (STV) has been adopted as a criteria for design in the most recent IESNA Recommended Practice for Roadway Lighting, but does not correlate with the night/day accident rates at speeds below 45 MPH. STV is currently calculated using a visibility model and the influence of only the fixed lighting system. When the effect of the vehicle headlights on pavement luminance, target luminance and glare are added to the calculation of target visibility, the criteria is named STV-H and correlation with the night/day accident rate is achieved at all vehicle speeds. In order to include the effect of headlights on target visibility it has been necessary to develop and validate algorithms for calculating pavement luminance at angles other than the conventional one degree viewing angle (alpha angle) from both headlights and fixed lighting luminaries. Correlation results using all currently recommended lighting criteria and STV-H are compared using night/day accident data from FHWA Report 92-R-00016.