"With high gas prices in the third quarter of 2005, national transit ridership grew by 3.3 % from the same period in 2004, according to a report released by the American Public Transportation Association (APTA) today." [LINK]
And the increase is even more in big cities (even LA!).
So with higher gas prices people take public transportation more. That decreases teh number of cars on the road which makes driving more efficient for those who do it, but decreases the petroleum products use by Americans. That decreases our oil needs, and also decreases the CO2 emissions.
Raising gas prices, through an at-the-pump tax, would then decrease traffic, decrease dependence on foreign oil, and decrease greenhouse gas emissions. The money raised by the tax could be used for better public transportation, providing a positive feedback, since better transport will attract more riders, further reducing traffic, oil dependency, and ghg emissions. Why aren't we doing this?