Let me guess
The half that agreed were those who do below average mileage. Either way, two things are going to happen:
1. Money will not go back into transport. It doesn't now so it's not likely to happen in the future. Road transport subsidises general government expenditure to the tune of billions.
2. It's not going to be cost neutral. IT systems and bureaucracy have to be paid for so if the total take is the same as now there will inevitably be less money for government to use.