I don't think it has to be doomsday. Our economy is large and robust, even minus cheap gas. I think autos for personal transit are doomed. We'll continue to use internal combustion engines for some commercial transport, and continue to use jet airliners with fossil fuel. Most likely we'll turn mainly to nuclear fission to replace petroleum and coal. We'll expand the power grid to power electric vehicles for freight and mass transit. But there will probably be a period when we won't have enough energy to power cars or car-like vehicles.
I predict bikes and walking will be very popular in urban and suburban areas. So many of us don't like buses and trains.