1 time tax works for now... If they popularize electric, that will change really quick... Because right now, sure it's just a few electric vehicles, tax it once. If they become a major player in the vehicle market aka pushing out gas, those 1 time items won't be enough to cover the fixing of roads.
Gasoline tax keeps happening when people use gas, ergo money for roads... When those people stop using gas, who will they look to tax?
The electric vehicle folks! Try and keep up
