Excellent idea. I hope this catches on and is implemented in the US. Having to make a decision about price should encourage the use of public transportation; people will have to think, "should I park at the train station and take the train downtown for $6, should I spend an extra hour in traffic and save the $6, or should I take the express lane and get there a bit faster than the train but for $35?"

It's already being implemented in the US: Interstate 35W south of Minneapolis has variable pricing with a target speed of 50 mph.

Public transit, carpools, and motorcyclists are all able to use the lane for free.

Video about the system at http://www.dot.state.mn.us/upa/animation/UPA_video.html

Yes. You forgot when option: "Or should I get three other people to get on my car and use the express lane for free."