Since low-income people tend to drive less than the average American, they pay less in gas taxes than average...This is, of course, exactly the wrong way to think about tax fairness. When we think about people's ability to pay taxes, what matters is not the dollar amount they pay but the percentage of their income that dollar amount represents. Gas taxes are clearly regressive in that low- and middle-income Americans pay a lot more of their income in these taxes than do wealthier taxpayers.
It's unquestionably true that the tax paid on a gallon of gas is the same for the poorest American as it is for the wealthiest. And that set amount of tax is a much bigger share of total income for low-income Americans than for the wealthy. That's what makes the tax regressive. Now, you can argue (I haven't seen stats on this) that low-income Americans are less likely to own cars, and that they are also less likely to own gas-guzzlers. This is probably true, and is what sets the gas tax apart from very regressive taxes such as the cigarette tax. But at the end of the day, these probabilities don't come close to offsetting the inherent regressivity of gasoline taxes.
Tierney is right about the positive social impacts of a gas tax hike. It would make most of us rethink our decisions about what we drive and how far we travel each day. But we have to recognize that without any offsetting low-income tax rebates, the burden of such a change would fall most heavily on low-income and middle-income Americans-- exactly the opposite of the progressive income taxes that the Bush administration has worked so feverishly to cut over the past five years.