Carpool lanes exist to reward those who work to reduce congestion and pollution with a faster trip. I know that's good every time I look out my window and can't see the hills for the haze. Some areas allow zero-emission-vehicles (electric cars etc.) to also use carpool lanes with a solo driver, reducing pollution if not congestion.
Proposals have been made to also allow solo drivers of hybrid cars into the lanes, as well as solo drivers who simply pay a fat fee for a permit. Let me propose an interesting variant of these payment ideas.