A method of passenger flow determination comprises estimating, by a processor (3,6), whether the number of mobile devices on a vehicle (2) has changed based on signals from mobile devices that were received at a sensor (1) located in the vehicle. The processor may compare an estimate whether the number of mobile devices on the vehicle has changed with at least one other data input to the processor, and this allows the processor to determine whether a change in number of detected devices is caused by someone entering or leaving the vehicle rather than a device being switched on or off.The processor may be located on the vehicle, and the method may further comprise forwarding information to a second processor (6) remote from the vehicle. The second processor will receive data from many vehicles and therefore has a larger dataset available than the processor on a single vehicle, and so can extract more reliable information about passenger flows.