In Singapore we have had electronic road pricing for over a year.
The charging rate varies with the time of day, and the rates are adjusted from time to time, in the light of average traffic speeds. Each car is fitted with a small, windscreen-mounted black box into which is slotted a stored value cash card. On driving under an overhead gantry the charge is deducted and the value remaining displayed. Should you omit to slot in your cash card or have insufficient value to pay the charge, a photograph is taken and the fine penalty sent to you by post. I have recorded a 100% success rate even in an extremely heavy tropical rainstorm.
Of course, in parallel with, and in advance of, the ERP, Singapore built a first class bus and Mass Rapid Transit train system on which one also pays by dedicated stored value cards which can be topped up electronically at numerous ATM machines.
Peter Dunlop (F), firstname.lastname@example.org