Shared taxis are ubiquitous throughout Southeast Asia. Unlike buses, this mode of transportation does not have fixed routes or schedules. Its distinctive feature is the transport of multiple passengers with different origins and destinations in one trip. Thus, shared taxis can be regarded as a public transportation service that is an intermediate between taxis and shared buses. More specifically, passengers inform the driver of their destinations, as in regular taxi services, with the driver picking up other passengers heading in the same direction along the way. If there are drop-off points for other passengers before they reach their destination, the driver makes a slight detour to drop them off first. Thus, travel distance can increase depending on the number of additional passengers along the path. Although the taxi visits the drop-off points of passengers in order of proximity, the drop-off points of any new passengers picked up during a trip are continuously added to the route. Consequently, passengers might not necessarily travel the shortest route to their destinations. In this study, we modeled a formula to determine the average travel distance of a shared taxi using Crofton's differential equations. We assumed that passengers appear randomly within a circular urban area and travel along a straight line between their origin and destination points. In our validation experiment, the average distance estimated by the model was compared with that obtained via computer simulation.