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.
Let people pay for part of their capool permit with used commuter train tickets. A person who rides the commuter train takes a car off the road just as much as a person who carpools. If used train tickets (for longer trips) could be credit for a carpool permit, this would encourage people to take the train "most days" but still use their car when it's called for. You could allow only redemption of your own ticket (such as a monthly pass) or any ticket, in which case a market would develop with people paying transit riders for their ticket stubs. This would effectively mean the solo drivers would subsidize the transit riders, even making their trips free. Which is part of what we want to have happen here.
There is a worry that selling access to the lanes for solo drivers would clog the lanes, making them worthless. To avoid that the price would have to be high. Even hybrid drivers could not get in for free if there are too many of them.
Instead, set a price for a carpool pass belonging to a specific vehicle. The price would depend on the EPA combined mpg of the car. An electric car could still get a free permit. A 60mpg hybrid might get an affordable permit. A 25mpg luxury sedan would pay a much higher fee. A Hummer would have to pay a truly exorbitant price. As noted, transit tickets could provide credit against the final price.
However, it's worth noting that if the lanes become so popular that's a sign that something is working, not a problem. The obvious answer is to increase the number of lanes. One could either add a 2nd lane, next to the left, over reduced hours, or create a judgement rule that says "2nd lane is Carpool or permit only when 1st lane is congested."
It's not like we have trouble telling when a lane is congested. We decide that 100 times a day while we commute, switching from lane to lane looking for the fastest trip. For legal purposes, cops could ticket solo drivers in the 2nd lane if the 1st lane is "bunched and moving 10mph or more below the limit." Not like we can't figure this out.
We could also sell single-day carpool permits (or give them to the transit riders to use or sell.) Such a permit would have a peel-off layer like an old Polaroid. Once exposed to air and/or light, it would turn black in a few hours.
Showing people a faster commute will get them to really work towards reducing our oil consumption, clearing our roads and reducing emissions.