I don´t know if this exists somewhere but i was thinking in a way to make taxis more efficient.
This would be an app sold to taxi companies, this app would acts like a gps of there taxis and tell them where they are and if they are available. This serves for taxi companies to control there fleet.
Now in the case you are an independent taxi you could buy this app to and have your taxi registered in the app.
Here is where this app starts working, costumers also download the app, the costumer version not the supplier. Every costumer could see where taxis are and if they are free. If you see a free taxi some blocks away you pinch it and the taxi driver will know immediately that he has been called, and where to go. This could make taxis more profitable as they make there time more efficient and costumers travel quicker.
This app could also leave the taxi requirement registered for safety of the taxi driver and the costumer.