In low demand areas, it has been proven that demand responsive transit (DRT) is more cost-effective compared to fixed route transit (FRT). The present study further examines the demand threshold of four public transit systems: DRT, FRT, fixed-route but non-fixed-point transit and dynamic stop transit, which are designated to feed urban rail transit. It constructs user cost models for these four transit systems, where we aim to find the system with the “best” user experience. The demand at which the user costs of two transit systems are equal is referred to as the critical demand. After deriving the analytical expression for the critical demand, a sensitivity analysis of the analytical solution is conducted. The results indicate that DRT is optimal for low demand density scenarios. If we allow more flexibility to transit services compared with FRT, fixed-route but non-fixed-point transit proves superior for higher demand. Additionally, the critical demands between DRT and FRT, as well as that between DRT and fixed-route but non-fixed-point transit, are influenced by the bus speed and passenger walking speed, where the impact has an upper bound; at the same time, the fleet size also affects the critical demands, where the impact has a lower bound.