I looked into this after they spent a billion in the north bay just to add empty carpool lanes. First of all, many people are unable to change their work habits to accommodate carpooling, and thus a major use of the carpool is simply bringing children to school. Since California is one of the only places in the world with carpool lanes, the studies mainly come from CA universities. Their studies point to a negative effect of carpool lanes on traffic in almost all traffic scenarios, because they are underutilized. Coupled with the fact that they add only negative value for every car that can access them without changing driving habits, there is a reason they have not caught on more widely. My personal evaluation was that they serve a political purpose rather than a practical one, and we pay hard cash for that.