Characterization of Log-Normal Distribution of Vehicle Spacing in Highway Tunnels with Implications for Evacuation Safety

Timely and efficient evacuation is crucial for tunnel fire safety. Previous studies often assumed fixed vehicle spacing and vehicle type ratios, which did not reflect real traffic conditions. Based on high-resolution satellite imagery of congested scenarios nationwide, 1703 sets of static vehicle spacing data were obtained. Descriptive statistical analysis and the K–S test were conducted. As a result, a log-normal distribution model of vehicle spacing was established, with an average spacing of 2.23 m, a logarithmic mean of 0.707, and a logarithmic standard deviation of 0.439. Analysis of 11,211 vehicle samples further revealed an uneven distribution of vehicle types across lanes. Based on the statistical data, a random vehicle arrangement algorithm was developed for highway tunnels, incorporating the log-normal characteristics of vehicle spacing. Using this algorithm, non-uniform vehicle layouts were generated for tunnel evacuation modeling. A total of 120 simulations across 12 different scenarios were then conducted to analyze the impact of log-normal vehicle spacing on evacuation time. Results showed that evacuation time was 2–5% longer under a log-normal distribution than under a uniform distribution. Therefore, future tunnel evacuation simulations should account for the log-normal distribution of vehicle spacing and the actual vehicle type ratio.