This paper proposes a low-cost but indirect system which can be used for occupancy detection and occupant counting. The system can serve as a way to confirm the number of occupants riding inside a car or the only way to determine the occupancy where the other means is not reliable (e.g., 2nd row occupants might not use seat belts and weight sensors might not be installed). The system can be used for various mobility applications including car rental, fleet management, taxi, car sharing, etc. The system utilizes existing motion sensors, such as those used for the Roll Stability Control function, together with door ajar signals. The motion signature during occupant’s on-boarding and off-boarding a car is first extracted from the response of the vehicle body which are reflected by the motion sensors. Then the weight of the occupant, during on-boarding and off-boarding, is determined through fitting the vehicle response signals to a transient response from a vehicle model. The effectiveness of system function in occupant detection and occupant counting has been tested on a test vehicle with various occupancy configurations.