I wondered the same thing. If you assume that the level of investment dictates the number of trips, then the ROI is steadily decreasing. Whether it's any worse than the ROI on, say, roads I have no idea.

My opinion, and I have no hard data to back it up, is that it's not a case of build it and they will come, but a case of they come, they complain bitterly about the lack of facilties and then and only then does it get built.