Because time cost forms a major part of the total cost of commuter transport, rapid modes will reduce real costs, but will also encourage longer commuting distances which will in turn increase energy consumption. While important, it is almost impossible to estimate empirically the scale of such effects due to the subjective nature of the value of time.
Given that time-related expenses make up a large portion of commuter transport costs, rapid modes will decrease actual expenditures but might also lead to longer commuting distances, thus increasing energy consumption. Despite their importance, it is nearly impossible to accurately measure the scale of these effects empirically due to the subjective nature of time value.