To be awkward, thanks to the Triple Convergence, rapid transit does almost nothing for drivers in the peak hour--trips just converge from other routes and other times. However, drivers on the shoulders of the peak (from whence trips migrate) do benefit. Rapid transit can still induce the long-cycle aspect of induced demand, when less-miserable commutes induces new development in peripheral areas. However, rapid transit continues to act as a congestion 'safety valve', such when the traffic congestion gets too bad, people switch from cars to rapid transit. But talking about rapid transit as a congestion reliever kind of misses the plot--parking is the real story, and rapid transit allows places to get denser (earlier, more cheaply) by reducing parking demand.