高速公路拓寬方案投資評估與決策分析

Abstract

本研究試圖提出一個高速公路投資決策模型，該模型可以評估未來高速公路可能的拓寬行為所帶來的效益，並將其效益於高速公路興建規劃時期就考量進去，以找出最佳的高速公路興建設計方案。本模型中，高速公路拓寬主要涉及三個不確定性因子，分別是：交通需求量、土地價格及營建物價指數。交通流量預測是高速公路評估過程中最重要的一環，高估交通流量會使決策者選擇興建較多車道，造成資源浪費；而低估交通流量會造成車道壅塞、用路品質滑落，以及其他的社會成本。土地價格變化對未來高速公路擴張方案有莫大的影響，如果預測未來土地價格很高或是取得本身就有困難，通常會現在先取得用地，以供日後擴張時之用。工程成本受國際物價波動的影響甚鉅，因此我們使用營建物價指數為指標作為工程成本的修正值，以找出同一工程於不同時點施工時的實際成本。在拓寬階段，興建成本除了本身興建工法選擇不同而改變，也會因用地取得的時機不同而改變。一般來說，如果沒有預留土地，日後擴張時要面臨工程複雜度會較高，興建成本亦會較高。常見的問題有管線遷移及鄰近用地補賞等。我們導入實質選擇權的方法來處理不確定性因子下的最佳決策問題，該問題的解決步驟是以蒙地卡羅模擬法為基礎所發展出來的。數值案例的結果顯示本研究所建議的模型與解決步驟是大有可為的。傳統的高速公路規劃是政策導向的，通常沒有明確定義要如何使決策最佳化，本研究為此最佳化問題提供了一個明確而可行的基本模型。

This research tries to present a highway decision model, which can evaluate the benefits yielded by the expansion of a highway in the future and consider this benefits in the design phase of a highway system to find out the optimal design alternative. In this model, the three main uncertainties involved in the expansion of a highway are namely, traffic demand, land price, and construction cost index. Forecasting the traffic demand accurately for the highway life cycle is an important task for economic purposes. Over-estimation of the traffic demand causes the waste of resource; under-estimation makes the highway service quality low and yields social cost. Acquiring the required land is needed for every highway expansion (widening) process. Land price and land availability are key points while facing the expansion of highway system. In expansion phase, the construction cost is affected by the construction cost index and the difficulty to road works as well. Real options are employed to solve the question of optimal decision-making under uncertainties. The solution algorithm is based on the Monte Carlo simulation. Numerical results show that the proposed model and solution algorithm are promising. Decision-making optimality is generally not well defined in traditional policy-based approaches for highway planning. This research provides a reasonable model and makes a radical step towards optimal decision-making in highway planning.