我國民法第787條、第788條、第789條之袋地通行權，第779條之過水權，第786條之管路通行權，第854條不動產役權之必要附隨行為之權，本質上為相似問題（本文稱之為「法定通行權」）。但文獻尚未將此六條文合而觀之，對各條背後立法緣由之分析，亦尚有未盡之處。本文運用法律經濟分析，探究此六條文背離財產原則，改採補償原則、甚至無償使用原則之經濟原因。本文主張，通行權為法定而非意定，是為了降低袋地所有權人與鄰地所有權人間之交易成本，以利兩造後續交易。法定通行權侷限在特定情境，而非一般性之規定，是要確保通行之邊際社會利益夠高。各條對「必要性」之要求，可以解釋成通行程度應該設在邊際通行利益大於邊際通行成本處。償金之規定是為了讓袋地所有權人內部化通行所造成之外部成本。但第789條採取無償原則，也有道理，因為該條適用之情境是土地所有權人可以事先安排通行處，採用無償原則可以降低事前的協商成本與事後的紛爭解決成本。「鄰地損害最少」只是四種典型標準的其中之一，且不總是最有效率。在特定條件下，「袋地所有權人自由選擇」反而會有助於社會福利極大。但現行規定也有時較有效率。從事前觀點，任意行為所造成的通行需求，不應該適用法定通行權之規定。

In the Taiwan Civil Code, six articles (779, 786-789, and 854) stipulate legal servitude of passage. Four of them address the access problem for landlocked land, whereas the other deals with drainage work that has to pass through adjacent land and ”incidental actions” by the owner of the dominant land. Generally, under certain circumstances, the Taiwan Civil Code prescribes that a landowner in need (often because of the landlocked nature of the plot) may use an adjacent plot, as long as the use is necessary and causes the least damage to the owner of the adjacent plot, who will be compensated by the landowner. Using economic analysis of law, this article examines the efficiency of the above stipulations, and makes the following claims. First, a statute-prescribed servitude of passage is necessary to reduce the asymmetrical bargaining power the neighbor has vis-à-vis the landowner. Second, the legal servitude of passage is available in only certain circumstances to make sure that the social benefit of passage is high. The necessity requirement can further ensure the social benefit of passage is larger than the social cost. In addition, the necessity requirement should be interpreted to prescribe that passage be set at a level where social benefit is larger than (rather than equal to) social cost, because the court can at best appraise the fair market value of the plot used for passage, but fair market value is lower than economic value, the true loss of the neighbor. A sub-optimal passage minimizes the neighbor's loss and at the same time reduces the transaction cost enough to pave the way for future voluntary transactions between the two parties, which generally increases efficiency. Third, the least damage rule is one of the four prototypical rules and is not necessarily the most efficient. Sometimes, the neighbor-name-it rule is more efficient. Nevertheless, overall speaking, the least damage rule has enough efficient properties to be kept. Fourth, from the ex ante viewpoint, the legal servitude of passage doctrines should not apply to voluntary landlocked land. Finally, the doctrine that is called ”statutory easement” in the U.S. makes economic sense, because it induces the relevant parties to make arrangement of passage beforehand and internalize the cost of passage.