Park-and-ride facilities are a valuable transportation systems management technique to combat congestion problems in and around city centers. It is therefore important for engineers and planners to determine their optimal location. Past research has identified many criteria that are relevant to optimal park-and-tide locations. But, although one criterion may carry more or less significance than another, they have always been considered on an equal scale. Furthermore, there is currently no system to use the existing knowledge and experience to aid planners in deciding the optimum location of a park-and-ride facility. In this project, a hybrid knowledge-based expert system/geographic information system tool was developed to help determine the optimal location for park-and-ride facilities. The tool was tested twice-on a set of hypothetical roadways and on another set of actual locations throughout the state of Delaware. In both cases, the program was successfully able to identify the same location that the selected expert engineers/planners chose as the optimal location for a park-and-ride facility given a set of potential sites.