I'm moving to SoCal and I'm thinking about getting the express lane fasktrak.. read up on it. First and foremost, it's a pilot program to see if it's effective at reducing commute times in majorly congested freeway areas. It's not permanent at this time.
There are three settings for the fasktrak:
1) Solo Driver
2) Two person car pool
3) Three+ person car pool (assumed the lowest rates as well)
You can absolutely leave your fastrak in mode #2, however there are cameras which will take photos every so often, along with check points in which your transponder will check in with fastrak for additional fees if any, not to mention in their video they mention the CHP very frequently, so I'm guessing they will heavily watch for violators as its free money for the entity.
I gather the violation will be equivalent or higher than the carpool violation fee ($280+?).
Also, if the speeds in the express lane(s) drop below 45 mph, overhead signs will warn you, and the lane will convert back to carpool only, until the speeds pick up again.