<p>Ridesharing allows owners of private cars to share these with other travelers, filling up vacant seats. Based on a real case from Norway, we define and study the Ridesharing Problem with Flexible Riders and Fixed Lines (RPFRFL), where flexible riders can act either as passengers or drivers that can pick up other passengers. Moreover, the RPFRFL also considers fixed (bus) lines, to which drivers can deliver picked up passengers at a terminal for public transportation to their final destinations. We propose a mixed integer programming model and a path flow solution method to solve this problem. Based on a large number of test instances for our case study, we study the effects of considering flexible riders and the integration with fixed bus lines and show, for example, that more than 10% additional passengers can be serviced when 25% of the riders are flexible compared to the situation without these. Furthermore, our analyses indicate that there is great value in integrating the ridesharing planning with public transportation, i.e., where passengers can be transported through ridesharing to a bus terminal for further transport. These results demonstrate that including the terminal delivery option gives 17.2% higher average number of serviced passengers compared to the case without. Furthermore, we analyze the riders’ willingness to participate in ridesharing under different conditions and compare the performance of our solution method with an existing ridesharing platform. The results show that our modeling approach and the path flow solution method have the potential to significantly improve the efficiency of the ridesharing system compared to the ridesharing platform currently in use.</p>