Yes, it is legal to be in the carpool lane. Even if u riding solo. ive done it wit cops behind me numerous times and havent been stopped yet. Ive even asked lots of cops too n they've said its all good. :thumbup:
It might be different from state to state, but this is some info I found.
Before the law was amended to include hybrids, HOV lanes were reserved for cars with two or more passengers during rush hour traffic: 6-9 a.m. weekdays for inbound traffic and 3-6 p.m. for outbound. Motorcycle riders are the only other solo occupants allowed in HOV lanes.