Quantifying Light Penetration in Short Tunnels: Development and Validation of a Numerical Model

Light penetration into a tunnel varies considerably with the site conditions as well as tunnel geometry. Short tunnels, defined as those less than 125m (410ft) in length, receive insufficient attention regarding their lighting requirement largely due to the misconception that natural light can penetrate through them, thus eliminating the need for artificial lighting. However, past studies and data reveal that this assumption may not necessarily hold true, with short tunnels experiencing higher crash rates compared to longer ones, partly due to visibility and driver’s adaptation to lighting conditions. Field observations further show that certain regions within short tunnels have insufficient lighting, making smaller objects difficult to see. Moreover, there is a notable lack of reliable national guidelines for providing supplemental lighting within short tunnels. This underscores the need to study light penetration conditions in short tunnels. In this paper, a numerical model was developed to address this need. The model is based on the inverse square law of light propagation and the Lambert cosine law of illumination, which serve as the foundation for simulating lighting propagation in this study. Our proposed numerical model can quantitatively determine the spatial variation of light illuminance within a short tunnel. The model’s outputs were independently verified and validated using field collected data. Consequently, with more studies and refinements, this model can potentially be further developed into a design and analysis tool in the future to establish lighting requirements and improve driver safety in short tunnels.