Earlier quoted context omitted.
Gas taxes set a floor for how low prices can go. They'll probably increase as gas motor cars are phased out before the process ever sink too low. In Washington, US, they increased the registration fee for electrics to make up for the shortfall in gas taxes that pay for road wear, congestion, etc. So that's already happening.
>They'll probably increase as gas motor cars are phased out before the process ever sink too low. Why?
I don't see diesel going away so soon though because of industrial, agriculture, and military use. It will probably be taxed differently according to it's use. In fact it already is taxed differently in the US for home heating, vehicle, and aviation use.