In this answer by Zach Lipton, it is mentioned that the price of using the express lane in California depends (mirror) on the number of occupants that the vehicle contains:
How do the Express Lanes know how many occupants a car using the express lane contains?
Relation to https://travel.stackexchange.com: it is useful for the traveler to know whether they have to make any kind of declaration regarding the vehicle occupancy.
As it says in the table, to get the discount, you have to have a FasTrak Flex or CAV transponder. These transponders have a switch that you set to indicate the number of occupants. The express lane sensors detect the setting and charge you accordingly.
The sensors also apparently transmit this information to police in the area, presumably paired with your license plate or other information identifying your car. If the police see that the number of people in your vehicle does not match, you can be pulled over and given a ticket. The fine can be over $400. As Alex says in the comment, there may also be cameras taking video which can be reviewed later to see if you have the claimed number of occupants, and they can ticket you by mail if not.
