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.
The demand side is where it gets tricky. If there are only a few gas stations in your state it doesn't matter that the list price if oil is cheap. Gas stations will tear out their pumps as demand drops.
