Driving into the city every day I often wish one lane was tolled. It's not that I want to use it necessarily, it's that it would let all the lanes flow well.
Imagine the motorways with overhead gantries on which a price is flashing showing the toll you will pay if you want to use a designated fast lane where three or four lanes permit one. The price would reflect the congestion of traffic on that stretch of motorway at the time. The heavier the congestion the higher the price.
You could be charged the rate applying at the time you decided to enter the lane and disregard any subsequent price flashing on the gantry. That would avoid the problem of drivers desperately trying to get out of the lane if the price went up. The camera on the gantry would record you only on entry, and the price showing would be all you would pay no matter how far you went. Simple, isn't it?
Right now when I'm in crawling traffic I know there will be a certain number of drivers around me fretting and fuming that they can go no faster. They may be just congenitally impatient but more likely they will have a good reason. Running late for something or anxious to be on time for an appointment. Whatever their reason, it is in the interests of all of us to let them pay for priority. It would reduce the traffic in the free lanes and we would move faster too.