Oregon's successor to the gas tax

Oregon has brought up a good idea for the eventual phasing out of the gas tax and phasing in a road user fee based upon the amount of miles. A transponder in your car would keep track of how many miles you drove on Oregon's roads and would transmit that data at the gas pump so you would pay the fee there rather than paying a gas tax. As a revenue source, the gas tax has failed to keep up with inflation as well as the increased fuel economy in vehicles lessens how much fuel people purchase. I think for this to take off, it would need to be implemented in at least a block of states to begin with or even nationally. Also, it sounds like their plan would be to include the transponder only in new vehicles and existing vehicles would just pay the gas tax. This way neither the DOT nor the consumer would have to pay to retrofit older autos. Anyway, more can be read about it here.