[hackerlight]

> the amount of gas purchased (and therefore taxed) is directly proportional to the negative externality of CO2 emissions.

This doesn't mean the gas taxes are big enough to correct the market failure. Your comment would be true even if we multiply gas taxes by 0.01, 0.1 or 10.0.

I would prefer it if we just used properly calibrated (larger) gas taxes as the single mechanism. But voters hate that, so we're left with second best solutions.