(for the umpteenth time...) In SoCal, Diamond Lanes (HOV lanes) were created, NOT to relieve congestion, but to get the South Coast Air Quality management District out of hot water with the Feds, because they flunked the Clean Air Standards year after year. The mitigation measure of HOV lanes was a means to placate the Feds so that SoCal didn't lose it's federal funding. Since the Prius is something like 10 times cleaner than a conventional car, it actually does more to achieve clean air than several 2-occupant carpools.
Like EVs, LNG, and other alt-fuel vehicles with low emissions, the Prius belongs there as an implementation of the purpose of the Diamond Lanes. Other states might have had other reasons, but avoiding loss of funding from air pollution standards failure was the motivation in SoCal.
Local drivers/voters HATE them, and in fact we have a local regulation that forbids conversion of any regular lanes to Diamond Lanes; only new, added lanes can be used for that purpose.
Clearly, an empty lane does nothing to reduce congestion. With a few exceptions, SoCal HOV lanes are wide open, so using them for ALL cars would instantly relieve some congestion. If theor main purpose was congestin relief, this conversion would have already happened.
While it's unrealistic to speculate along the lines of "if everyone drove a Prius...", the benefits of cleaner air, the intent of the lanes in the first place, are served by encouraging more cars like the Prius. For this reason, our local legislators want to let California decide what's best for California roads, not Washington DC.
Keep in mind the Cali rules stipulate onlt 75K permits and a sunset in 3 years, so this is clearly an incentive program for clean air (a benefit which continues past the 3 years), not a permanent policy to give special treatment to hybrid owners.