A gas tax is generally ineffective, mainly because we're dealing with inelastic demand here. Gasoline is about on top of the list of examples of inelastic demand in a microeconomics textbook.
Most people do not wake up each morning saying to themselves, "So how can I destroy the environment today?" Most people drive out of necessity. Sure, I lived in a large city for a couple of years, but I can certainly see why people flee them. Between exorbitant housing costs, whether through rentals or mortgages; city taxes (where applicable); and/or jobs that do not pay relative to the cost of living, I can see why people flee cities.
The solution to this problem is not taxation--which will do little to curb inelastic demand, short of busting the economy completely--but through gradual regulation. Sure, it is not necessarily the place of government to interfere, but they certainly have the power to encourage and punish business practices as they see fit. If the government wants more E85 cars, for instance, they could certainly force the issue, much as they forced the issue of fuel efficiency in the 1970s or the elimination of leaded fuel in the 1980s.