一直以來，許多學者皆指出公共財具有外部性的特質，且部分公共財的外部性 會隨著距離的加大而逐漸減小，然而，到目前為止，文獻上卻少見關於外部性距離 衰減的相關研究。一般而言，都市公園綠地是一項具有外部效益的公共財，其外部 效益沒有實際的交易市場，因此我們無法直接取得此一結果，但都市公園綠地的外 部效益可資本化於周圍的房屋價格之中，成為房屋的屬性(特徵)之一。藉由此一特 性，本文應用特徵價格理論，透過迴歸分析，建立臺中市房屋特徵價格函數，藉此 估計臺中市公園綠地外部效益之距離衰減函數。 研究結果發現，若臺中市房屋特徵價格函數設定為雙邊對數函數，則至公園距 離增加時，房屋的成交價確實會下降，而且隨著距離的增加，成交價下降的幅度將 逐漸減緩，表示都市公園綠地的確具有外部效益，且其外部效益會隨著距離的增加 而逐漸減小。此結果亦可顯示，都市公園綠地的外部效益在空間上確有一定的範圍 或局部性。

Studies have showed that public goods have the characteristics of externalities and externalities of certain public goods diminish as distance from the locations of those public goods increases. However, few researches have focused on distance-decay of externalities. It is widely accepted that urban parks are public goods with external benefits. Since there is no physical market for the external benefits of urban parks, we cannot obtain the results directly. Nevertheless, the external benefits of urban parks can be capitalized in housing prices as one of the attributes of the property. Based on this characteristic, this study applies hedonic price theory and regression analysis to establish the hedonic price function of houses in Taichung city. This function is then used to estimate a distance-decay function of external benefits of parks in Taichung city. The results show that if the hedonic price function of houses in Taichung city is specified as a log-log function, as distance to the closest park increases, the prices of houses decrease with a decreasing rate. It implies that urban parks do possess external benefits and external benefits diminish as distance to the park increases. The results also suggest that the external benefits of urban parks have limited spatial (geographical) extent and, therefore, are localized.