The Guardian reports on an upcoming, government funded, report in the U.K. that will recommend a new system to price road access. It sounds like cars would be required to carry a combination GPS/radio unit capable of transmitting the cars' locations to satellites. The system could track road usage and bill drivers appropriately. The report is due out later this month. The goal is to address road congestion problems. Here's the story: "Crisis plan for tolls on all roads ".
It sounds like, with this technology, it would be relatively easy to adjust charges to reflect varying levels of congestion in time and space (downtown at 3:30 PM on Sunday morning may be relatively uncongested and the charge per mile could be low; the inbound expressway at 7 AM on Monday morning may be relatively congested and the charge per mile could be high).
The technology sounds like that used to monitor vessel location and movement in the waters off Alaska. Certain classes of vessels are required to carry Global positioning system (GPS)/radio units so that their movements can be monitored. This system was introduced to monitor vessel activity with respect to areas closed to protect the endangered Stellers sea lion.
I learned about the Guardian article from Skip Sauer's Sports Economist blog: "Toll Roads"