雪山隧道之速限對通行能力的影響分析

IMPACT ANALYSIS OF TRANSPORTATION CAPABILITY BY SPEED LIMIT IN XUESHAN TUNNEL

雪山隧道的行車速率管制，自通車以來由原先的速限70公里／小時，調高到80、90公里／小時。然而，雪山隧道的速限調整，主要著眼於運輸通行效率的提升，但是否有顯著效果？本研究利用交通部高速公路局的VD資料，分析雪山隧道內的自由車流速率，以了解小車在隧道內運行之基本特性。其次分析隧道內的速率與流率關係及容量，並進一步剖析速限提高對於車道容量與通行能力之影響。結果發現，速限從70公里／小時調高為80公里／小時之後，容量顯著增加，北上內外車道平均約增加200輛／小時，南下內、外車道平均各約增加150輛／小時及300輛／小時。但速限調高到90公里／小時後，容量沒有顯著變化。然而，速限提高到90公里／小時之後，雪山隧道在不同需求流量狀況下之里程生產量普遍提高；在流率接近容量時，里程生產量約提高10%，顯示運輸能力有所提升。

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.