Small target visibility is widely used to evaluate the quality of road lighting. It provides a link between lighting design and driving performance. However, it is based on a strong simplification of the driving task using psychophysical data from laboratory conditions. Using a driving simulator to mimic the real driving environment, the impact of driving workload on target detection performance in mesopic vision conditions has been evaluated. The target visibility level is studied with and without driving workload together with different luminance contrasts and target positions, with reference to the small target visibility scenario. The results show that the driving workload significantly reduces the target detection performance. Consequently, the visibility level value for driving conditions should be much higher (visibility level ≥21) than some currently recommended ones (visibility level = 7) to achieve the same detection rates. Effects of target position and contrast are found in a way consistent with the literature. In addition, results indicate that the small target visibility model used for road lighting is limited and needs to be improved for a reliable prediction of visual performance with driving workload.

This article concerns driving after dark. Control of driving speed, being one aspect of the cognitive load of driving, is an important factor which influences visual performance when driving. However, it is not included as a parameter in the widely used models, such as small target visibility and relative visual performance. A driving simulation platform was established to enable investigation of the effect of driving speed on target detection performance. In addition, the effects of target contrast, position and the initial target appearance distance were also evaluated. The results show that increasing driving speed leads to a significant reduction in both the detection rate and the detection distance. The effect of speed was characterised using the Visibility Level model, by calculating two boundary values of Visibility Level for different speeds. The results show that driving speed has an impact on target visibility. A new model is proposed, the Simulator-based Visual Performance model, the metric for which is the product of detection rate and detection distance. It describes the effect on visual performance of driving speed, target contrast, distance and their interactions. The Simulator-based Visual Performance model predicts the experimental results well.