Road-Surface Contrast and Adaptation of Visibility of Low-Contrast Targets

This study investigated the visibility of low-contrast small targets on a roadway illuminated with light-emitting diode (LED) lighting. Detection-distance measurements were taken on two pavement types, with multiple roadway-lighting illuminance levels, and with and without headlamps. Contrast, vertical illuminance, and visibility levels were also measured and/or calculated. Results showed that the uniformity of the pavement affected mean detection distance. Concrete pavement had less luminance uniformity and longer mean detection distances, while asphalt pavement had more luminance uniformity and shorter detection distances. When headlamps were on, mean detection distances were shorter, possibly because headlamps create a brighter environment to which the eye adapts, lowering its sensitivity to low-contrast objects. When the vehicle was close to the target, the headlamp light could drown out the contrast between the target and background, making it difficult to detect. In general, the brighter the LED roadway lighting, the longer the detection distances, but the results were not significant in three out of four experimental conditions. Visibility level, calculated using Weber contrast and a number of other factors, did not accurately predict the mean detection distance for two experimental conditions. Those calculations might not fully describe small-target visibility for all roadway conditions.