Motorcycles are permitted in High Occupancy Vehicle (HOV) lanes in all fifty states and no highway with any federal funding may restrict access to motorcycles. From 23USC102:

TITLE 23--HIGHWAYS
CHAPTER 1--FEDERAL-AID HIGHWAYS
SUBCHAPTER I--GENERAL PROVISIONS
Sec. 102. Program efficiencies

 (a) HOV Passenger Requirements.--
    (1) In general.--A State transportation department shall 
establish the occupancy requirements of vehicles operating in high 
occupancy vehicle lanes; except that no fewer than 2 occupants per 
vehicle may be required and, subject to section 163 of the Surface 
Transportation Assistance Act of 1982, motorcycles and bicycles 
shall not be considered single occupant vehicles.
    (2) Exception for inherently low-emission vehicles.--
Notwithstanding paragraph (1), before September 30, 2003, a State 
may permit a vehicle with fewer than 2 occupants to operate in high 
occupancy vehicle lanes if the vehicle is certified as an Inherently 
Low-Emission Vehicle pursuant to title 40, Code of Federal 
Regulations, and is labeled in accordance with, section 88.312-93(c) 
of such title. Such permission may be revoked by the State should 
the State determine it necessary.

(b) Access of Motorcycles.--No State or political subdivision of a 
State may enact or enforce a law that applies only to motorcycles and 
the principal purpose of which is to restrict the access of motorcycles 
to any highway or portion of a highway for which Federal-aid highway 
funds have been utilized for planning, design, construction, or 
maintenance. Nothing in this subsection shall affect the authority of a 
State or political subdivision of a State to regulate motorcycles for 
safety.