Why are motorcycles allowed in some HOV lanes?
Motorcycles are permitted by federal law to use HOV lanes, even though they typically carry only one passenger. The explanation for the federal law is that allowing motorcycles to use HOV lanes keeps them moving, and it is considered safer to keep two-wheel vehicles moving than it is to have them traveling in start-and-stop traffic conditions. The individual states can choose to override this provision of federal law, if they determine that there is an inherent safety risk by allowing motorcycles to use HOV lanes. In the State of California, motorcycles are permitted to use HOV facilities unless a traffic control device specifically prohibits them.
According to US DOT Federal Highway Program's website, the "primary purpose of an HOV lane is to increase the total number of people moved through a congested corridor ", and they are also an "environmentally friendly option" .
However, here's an interesting fact: Contrary to what a lot of us believe, regular motorcycles are not more environmentally-friendly than cars.
California's Air Resources Board has recently imposed standards  that cut emissions of motorcycles closer to car emission standards. Indeed, without these standards, "motorcycles now produce up to 15 times the emissions per mile as the average new car or light-duty truck" (also in ), despite getting better mileage. Some motorcycles have catalytic converters and other features which would vastly reduce emissions below car level, but they are not required. The EPA has followed suit in 2005 with similar NOx and HC restrictions  starting in 2006 and lowering again in 2010.
In related news, the state of California is the only state to allow motorcycles to lane-split, that is, motorcycles to pass between the lanes of congested traffic.
With the signing of the federal transportation bill by Pres. George Bush in Aug. 2005, states were allowed to issue stickers to owners of hybrid vehicles, allowing them to drive solo in HOV lanes.
Arizona, California, Colorado,Florida, New Jersey, New York, Tennessee, Utah and Virginia allow hybrids in HOV lanes.
Several of these states regret issuing the thousands of tags to hybrid cars as HOV lanes are getting over-crowded.
 US DOT Federal Highway Program, http://www.fhwa.dot.gov/operations/hovguide01.htm
 California Air Resources Board, http://www.arb.ca.gov/msprog/motcycle/onrdmc.htm?PF=Y
 EPA, here