There are instructions for how to apply them. They're not particularly unobtrusive, unfortunately. Some people have chosen to not follow the instructions, and have applied only a single sticker on the back (Elon's car is like this).
They have no standing outside of California.
The installation locations are shown on this page (scroll down a ways):
Clean Air Vehicle (CAV) Decals - High Occupancy Vehicle HOV Lane Usage
This article has a photo that shows Elon's car, and how the sticker has been applied on it.
Tesla Model S: L.A. to Las Vegas on a single charge | SmartPlanet
They have no standing outside of California.
The installation locations are shown on this page (scroll down a ways):
Clean Air Vehicle (CAV) Decals - High Occupancy Vehicle HOV Lane Usage
This article has a photo that shows Elon's car, and how the sticker has been applied on it.
Tesla Model S: L.A. to Las Vegas on a single charge | SmartPlanet