Here around Denver, we have the ExpressToll system, which started out as the electronic billing system for E-470 (the toll-only highway that runs around the eas

Here around Denver, we have the ExpressToll system, which started out as the electronic billing system for E-470 (the toll-only highway that runs around the eastern Denver metro area), but then got adapted to the I-25 express lanes, and then to HOT lanes carved out of existing freeways (US-36 and northern I-25 first, then C-470, I-70 through part of the mountains, and more coming soon). They use either a basic RFID sticker that always charges the toll, or a switchable pass with two positions. One bad thing: "high occupancy," for which you can use the "free" position on the switch, requires 3 occupants now, not just 2. The other toll-only highway, the Northwest Parkway, has its own pass system (GO PASS), but ExpressToll is compatible with it as well. I have the sticker for my car; it's always seemed to work OK for me.