Determining occupancy of a vehicle during a trip can be carried out using a computer server which receives data from mobile computing devices within the vehicle. Each of the mobile computing devices is associated with a person, and sends to the server a unique identification of the vehicle, such as a license plate number, and also data generated during the trip from sensors within the device. The sensors collect data that relates to the local ambient environment of the device during the trip, such as a local magnetic field, movements, altitude, location, and sounds. The server compares the data from all devices in the vehicle to determine if the data from all of devices match, within predetermined limits. If there is a match, the server can provide a probable vehicle occupancy count; otherwise, the server can provide an indication that the vehicle occupancy should be investigated in another manner.