Model of Volume-Delay Formula to assess travel time savings of underground tunnel roads

This article presents the result of conducting a working model of the Volume to Delay Formula (VDF) that will be applied to analyze the economic feasibility of underground tunnel road projects in Seoul. Without this formula, highway engineers are unable to assess the travel time savings of the underground tunnel road projects, and this troubling situation deters a reasonable progress of this massive plan. To deal with this problem, the research carried out a field survey study in an existing mountain tunnel to capture the characteristics of tunnel traffic flow and conducted a follow-up study to calibrate operational parameters of this tunnel traffic flow. The Golden Section search method was adopted in this purpose and major findings revealed that a reliable VDF could be successfully developed based on the analysis of existing mountain tunnel flows. It was also found that traffic volume levels estimated with existing VDF's and the ones estimated with developed VDF's were much different: a more reasonable volume level was observed across all highway links. We thus concluded that the developed VDF's should be applied to assess travel time savings of the underground tunnel road projects in Seoul. The study results can be applied successfully to six lane tunnels, but more validation efforts are required for other types of tunnels.