I think using GPS in place of ERP is in the pipeline. With this, can remove COE totally. What will happen is when you drive, you will pay. The more you you drive, the more you pay and driving on congested roads will need to pay a whole lot more.
This will make owning a car relatively cheap but driving one will be very very expensive. This way can encourage people to use the public transport,(or walk or cycle) car pool and use the car only when really needed or only during off peak hours and using less congested roads.
the last time the GPS system was mooted, wasn't there concerns over privacy?