Institute of Transportation, MOTC

　　The speed limit in Xueshan Tunnel was 70 km/h in the beginning, and was raised to 80 km/h and 90 km/h primarily for improving the traffic operating efficiency. But, were there any significant effects? This study used the VD data from the Freeway Bureau to investigate the free flow speed in Xueshan Tunnel and identify the basic operational characteristics of small vehicles. The relationship between the speed and flow-rate and the capacity in the tunnel were then analyzed, followed by an investigation of the impacts of raised speed limit on the capacity and the transportation capabilities. The results showed that the capacity significantly increased to an average of approximately 200 veh/h on the northbound lanes, 150 veh/h on the southbound inside lane, and 300 veh/h on the southbound outside lane when the speed limit was raised from 70 km/h to 80 km/h. No significant changes in the capacity were observed when the speed limit increased to 90 km/h. However, the mileage production at various flow rates generally increased after the speed limit was raised to 90 km/h. The mileage production increased by about 10% when the flow rate is close to the lane capacity, indicating that the transportation capabilities were improved.