中央分隔帶缺口封閉對用路人之影響

Abstract

道路路面較寬的雙向多車道上，為達管制交通、增進行車效率及改善用路安全等目的，會增設不可跨越式之中央分隔帶。道路規劃為了提供汽車迴轉及行人穿越，必須在分隔帶上設置缺口，但任意地設置缺口會嚴重影響車流之流暢度與安全性，增加對撞機率，容易造成致命性事故發生。立於交通工程安全規劃角度上，為確保民眾以及用路人安全，必須減少路面上缺口設置的數量，但大部分的民眾會因自身便利性受損而對於政府機關將缺口封閉之作法不表以支持。缺口封閉將導致車流無法於缺口處左轉及迴轉，車流必須改變行車路徑至鄰近路口轉向。本研究為一個案研究，針對選定缺口之路網幾何形狀進行分析與歸納，以學理邏輯與分析假設，找出對該缺口具轉向需求之車流可能改變的行駛路徑，估算缺口封閉後車流之轉彎次數變化、繞道距離與繞道成本。結果顯示對於大部份用路人之單一次旅程而言，三種指標值變化佔全程比例非常低，顯示缺口封閉對用路人之影響程度並不如一般想像高。而對於社會成本而言，若使用該缺口進行轉向而產生易肇事衝突之車輛數非常高，其引致之總社會成本並不容小覷，但相較於缺口封閉後可減少的交通事故損失，其淨效益仍相當明顯。

For the purpose of vehicle movement control, enhancement of vehicular efficiency, and improvement of road safety, raised median could be provided on two-way multilane roads. Median openings should be implemented for U-turning vehicles and pedestrian crossing. If the median openings are implemented arbitrarily, it could affect the traffic-carrying capability and road safety seriously, increasing chances of head-on crashes, and resulting in severity accidents. On the standpoint of traffic engineering safety scheme, we should reduce the amount of median openings to assure the safety of road users. However, most of the road users do not support the measures that the authorities prohibit median opening movements, because it would diminish their own convenience. The closure of median opening would block the traffic to turn left and make a U-turn. Drivers must change driving routes to turn at the adjacent intersections. This study is a case investigation, in which the network of a study site was well analyzed and generalized, logic and assumptions were established to find the possible alternative traffic routes. The variation of turning frequency, detoured distance, and detoured costs were estimated assuming the closure of the median opening. The results showed that changes of the three indices are very low in terms of relative proportion compared to a selected journey for most road users. It indicated that the effects of prohibiting median opening movements for most road users are minor. If the turning vehicles at the median opening are to a substantial amount, the total social costs should not be underestimated, and the cost savings of reducing traffic conflicts and crashes by blocking the median opening are still in a great value.