If every cab would be like this, you would be home in a jippie.
You are driving a cab and to make money you have to transport people or packages from point A to B. If you do this fast you get more. When you have a nice amount of money you can go to the arms dealer and attach some weapons on your cab to shoot your way trough to "point B".
If you have enough money you can buy yourself a ticket to the next city where new and stronger opponents are waiting for you.