How do carpool lanes in the US work?

I'm doing some research for our roadtrip through SW USA (+Calif.) and ran into "carpool lanes" and this came to my attention:

Hopefully if you're not travelling alone, you will be allowed to drive through carpool lanes that help go past single-occupancy traffic

Since we're travelling with 2 (and part of the trip with 4), is it safe to say I can always use the carpool lanes? If more than 2 (e.g. 3) persons are required, is there a clear traffic sign that states this or do I have to know the rules in each state/city by heart?

I also found that 2 freeways in LA County (I-10 and I-110) were converted to toll roads and require RFID transponders. I suppose a rented car doesn't have a transponder? Is it made clear by any sign that you need a transponder?

Pictures about "How do carpool lanes in the US work?"

How do carpool lanes in the US work? - Roll of american dollars tightened with red band

How do carpool lanes in the US work? - Pile of American paper money on black surface

How do carpool lanes in the US work? - Smiling Woman Sitting Inside Car

What does 2+ HOV mean?

Occupancy requirements on HOV facilities are usually set at HOV 2+. HOV 2+ means single-occupant vehicles (SOV) pay toll to use the managed lanes. Unrestricted Use means ALL vehicles must pay to use the managed lanes.

What is the difference between express lane and carpool lane?

Express lanes are specially designated highway lanes that offer a new and faster commute option for highway drivers. For solo drivers who cannot use the high-occupancy vehicle (HOV) lane (also called a "carpool lane"), express lanes give the option to pay a per-use toll to use the lane.

What is the point of the carpool lane?

High-Occupancy Vehicle (HOV) lanes, also known as carpool or diamond lanes, is a traffic management strategy to promote and encourage ridesharing; thereby alleviating congestion and maximizing the people-carrying capacity of California highways.

What states have carpool lanes?

State HOV Lane RegulationsStateExemption TypeEligible VehiclesMarylandHOV ExemptionPEVNew JerseyPartial HOV ExemptionHEVNew Jersey10% HOT DiscountFuel economy greater than 45 mpg and meets California Ultra Low Emission Vehicle standardsNew YorkHOV Exemption and 10% HOT DiscountHEV, PEV10 more rows

How Carpool Lanes Make Traffic Worse - Cheddar Explains

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: Karolina Grabowska, Karolina Grabowska, Karolina Grabowska, mentatdgt