From the California Vehicle Code:

CVC Section 21655 States:

21655.5. (a) The Department of Transportation and local authorities, with respect to highways under their respective jurisdictions, may authorize or permit exclusive or preferential use of highway lanes for high-occupancy vehicles. Prior to establishing the lanes, competent engineering estimates shall be made of the effect of the lanes on safety, congestion, and highway capacity.
(b) The Department of Transportation and local authorities, with respect to highways under their respective jurisdictions, shall place and maintain, or cause to be placed and maintained, signs and other official traffic control devices to designate the exclusive or preferential lanes, to advise motorists of the applicable vehicle occupancy levels, and, except where ramp metering and bypass lanes are regulated with the activation of traffic signals, to advise motorists of the hours of high-occupancy vehicle usage. No person shall drive a vehicle upon those lanes except in conformity with the instructions imparted by the official traffic control devices. A motorcycle or a mass transit vehicle may be operated upon those exclusive or preferential use lanes unless specifically prohibited by a traffic control device.
(c) When responding to an existing emergency or breakdown in which a mass transit vehicle is blocking an exclusive or preferential use lane, a clearly marked mass transit vehicle, mass transit supervisor' s vehicle, or mass transit maintenance vehicle that is responding to the emergency or breakdown may be operated in the segment of the exclusive or preferential use lane being blocked by the mass transit vehicle, regardless of the number of persons in the vehicle responding to the emergency or breakdown, if both vehicles are owned or operated by the same agency, and that agency provides public mass transit services.
(d) For purposes of this section, a "mass transit vehicle" means a transit bus regularly used to transport paying passengers in mass transit service.
(e) It is the intent of the Legislature, in amending this section, to stimulate and encourage the development of ways and means of relieving traffic congestion on California highways and, at the same time, to encourage individual citizens to pool their vehicular resources and thereby conserve fuel and lessen emission of air pollutants.
(f) The provisions of this section regarding mass transit vehicles shall only apply if the Director of Transportation determines that the application will not subject the state to a reduction in the amount of federal aid for highways.

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