The taxes would go into effect only as prices fall. That would keep the price “artificially” higher and discourage consumers from waiting out the peak. The idea is that if you KNOW that prices will stay high then you’ll buy that economical car and move back in towards the city.
NOTE: I heard this on NPR, but can’t find the reference. It was commentary about missed opportunities from past economic problems. In this case, it hind sight from the 70s oil crisis.
Also, this is not NPR - it’s commentary about NPR commentary. Basically talking about fuel prices needing to be $7/gallon before consumers change their car buying habits.