In an Institute of Transportation Studies report to state and federal transportation officials, CEE Professor Mike Cassidy and Transportation Engineering graduate student Kitae Jang state that denying hybrids access to high-occupancy lanes seems to slow down traffic on the freeway over all — the carpool lane included.
See:
Booting hybrids from carpool lane backfires (SF Chronicle)
Ban Hybrids from the fast lane and everyone slows down (NY Times)
Kicking hybrids out of carpool lanes backfires, slowing traffic for all (Berkeley News Center)