Originally Posted by
docbert
You DO need a Fastrak (of any form) to drive in an ExpressLane. If you have a standard Fastrak then you will be charged to use the ExpressLane, regardless of how many people are in your car. If you have a Fastrak Flex, AND you have sufficient people in the car to meet the carpool requirement, then you can set the Fastrak Flex to the number of people in the car and it is free.
What you described is correct in the Bay Area, but in San Diego you can drive in the express lanes with no transponder as long as you have the requisite number of passengers to be considered HOV: http://511sd.com/fastrak511sd/how-to-use-the-I-15-Express-Lanes