Excuse my ignorance please, but I live a couple thousand miles away from California and we don't have HOV lanes are here. But why the heck does the state require you to put four big ugly stickers all around your car in order to drive in the HOV lane? Why wouldn't one sticker on the license plate or rear window be sufficient? If I'm driving my tesla from out of state and don't have stickers on my car, am I not able to use the HOV lane? All of this makes no sense. Have the citizens complained about these rules or does everyone just happily plaster these stickers around their cars?
I agree that putting FOUR stickers is excessive and ridiculous. Hence why I have not put any on my car. Although I believe Northern California may have some automated reading system(s) in place on certain roads.
The HOV lane concept originated a while back as a way to encourage using carpools and buses, ostensibly so that we would reduce traffic and decrease pollution.
The rules vary depending on which highway you are on and they vary significantly between Northern and Southern California.
You can read all the rules here. The sticker came about when low or no emission vehicles started showing up. Since they help with the pollution part of the reason behind the lanes, the state wanted to further incentivize people to get these cars. In California, HOV lanes can be VERY helpful - in pre-Covid times, it would reduce my 22 mile commute from 45-60 minutes to 25-35 minutes. YMMV
Technically speaking, you cannot drive in the HOV lanes as a solo driver without a sticker. Enforcement varies and is of course a crap-shoot. I have 17,000+ miles and would say easily half have been in the HOV lane and have not had an issue. But I also know I am gambling here.
On most Southern California HOV lanes, it only takes two (driver and passenger) to qualify as a carpool. Other places, it is three.
To further make driving confusing in the Golden State, there are also many toll roads where you need a Fastrack account. This is completely separate from the HOV lanes.
Finally, HOV lanes are also called Carpool lanes and Diamond lanes (the symbol used to designate the lane is a diamond).