In California, in San Francisco, they replaced a 3-person carpool lane with a 2-person carpool lane, because it was vacant most of the time. Makes sense, right? Lower the limit, more people will carpool, and the lane will be filled up with carpoolers!
This is exactly what happened on the first week. One the second week, all the new carpoolers realized that the carpool lane was just as congested as a regular traffic lane, so there was really no point in carpooling anyway. And fewer people carpooled until the lane reached some kind of equilibrium, where it made sense for people to carpool again.
So, no problem, right?
Wrong. One lane of 2-person carpoolers is worth as much lane capacity as two lanes for regular cars. One lane of three person carpools is worth three lanes of single-rider cars. Switching from a 3-person to a 2-person carpool effectively sacrifices a lane.