Well, you could do that, and you could also have a tiered rate based on vehicle weight or any other criteria you care to dream up.
I agree with you that this is less about revenue that it is about implementing the "green agenda". People are fed up with tax increases and will be uh, reluctant to increase the gas tax unless and until all of it is spent on road construction and maintenance, which these people don't want. That's the last thing they want, in fact.
The power to tax is the power to destroy.
They started researching this idea two years ago here in Oregon.