Technically, yes. IMHO, it depends on what toll roads you travel. Check this thread for some answers about the 10 and 110 lanes where you must have the DMV CAV stickers to get the three person toll free rate when driving solo:
Model X Windshield Mounted Sunpass (toll payment device)
Here is some text from 91 Express Lanes about the subject:
Do I have to mount my 91 Express Lanes transponder on my windshield?
Yes. The California Vehicle Code requires that a transponder be properly mounted on the vehicle's windshield. Keeping the transponder hidden may result in transactions being processed as violations and a citation issued by the CHP.
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It's a three person car for solo drivers only on the 10 and 110 freeways with the 3 person transponder
and you have the DMV CAV stickers. (Not sure about NoCal locations, check with the agency that runs your toll roads for details.)
Just don't try driving solo and use the 3 person lane on the 91 Express Lanes. The 3 position switch isn't used on that toll road. The only way to get a free or discount solo driver toll on the 91 and no monthly transponder fee is to apply for a Special Access account for ZEV vehicles when obtaining a FasTrak for all roads in California. It's only available at this site:
https://www.91expresslanes.com/signup.asp - Click on "Download Application". (You can also have 3 people in your car for the free or discount toll. Look for the small print in the FAQ section.)
Account holders with three or more people traveling in their vehicle travel toll-free except Eastbound, Monday through Friday from 4-6 pm. During this peak time carpools of three or more receive a 50% discount on the posted toll.