Why not consider multiple exposures to eliminate people. Open the shutter when there are no people in the scene and close it when there are. All you have to do is keep a record of the time exposed in your head, it can be fun. With a little patience you can be creative and have ghosts almost where you wish. Also, if a person is walking across the scene they are unlikely to register but if the are walking toward or away from the camera they will register albeir slightly blurred.