You already need to pay a toll to get into Manhattan, across every bridge and tunnel, maybe with the exception of one or two local bridges way up north. So, 'they' already know you are on the island; this just would help narrow down whether you were below 86th. And not very well-they probably won't record when you left. Given that prime Manhattan parking space is as expensive per square foot as prime manhattan real estate, and most traffic in Manhattan is cabs, I doubt this will have the impact they want. It seems to me the biggest impact will be for people who regularly make the trip across 86th street, which might cut down traffic on the George Washington Bridge, and might reduce traffic above 86th, and might get people to take a cab to 86th street and then transit the rest of the way home. It will bring in revenue, which is really what this is about.