Now seems like a good time to remind you of the incentives created by per gallon versus per mile gas taxes

From today's Washington Times:

An on-again, off-again move by the Obama administration to scrap the federal gas tax in favor of a pay-per-mile fee would boost the tab to Americans as high as 250 percent, raising their current tax of 18.4 cents a gallon to as high as 46 cents, according to a new government study.

But without a tax increase, said the Government Accountability Office study, the government's highway fund is going to go dry. One reason the fund is going broke: President Obama's push for fuel efficient cars has resulted in better mileage, and fewer stops at the pump.

The GAO study is just the latest review of federal spending that paints a grim picture of the nation's infrastructure. Just keeping spending at current levels, the GAO said, would require a near doubling of the gas tax to 32 cents a gallon, and that would jump to as high as 46 cents should the federal government add spending to fix crumbling infrastructure and build new roads.

The average driver pays about $96 a year in federal gas taxes, said GAO. Should the administration seek to raise the highway trust fund from $34 billion to the $78 billion needed to fix and maintain roads, that could rise to $248. Translated into a pay-per-mile plan, drivers would face a tax of 2.2 cents per mile compared to the 0.9 cents they pay now. Trucks would pay far more.

But isn't driving less (and thereby reducing the externalities associated with driving) the real goal?  Or is the goal to raise revenue?  If these seem like conflicting goals--they are.  As I noted on May 20, 2011:

...with a per gallon gas tax, total taxes paid will increase with miles driven and decrease as fuel efficiency increases.  The gas tax creates the incentive to Drive Less! and drive more efficiently. 

And a gas tax can't be manipulated as easily [as a mileage tax].

It is also worth noting that a [per mile] driving tax and a [per gallon gas tax] are not equivalent.  A driving tax is a flat rate tax per mile driven for all cars.  The driving tax creates no incentive to increase fuel efficiency (and possibly a disincentive) since increases in fuel efficiency will not reduce miles driven (and may increase them).

The administration's choice of policy will reveal the true motivation behind a gas tax:  If the motivation is to create incentives for better fuel efficiency and reduced externalities, then we will see an increase in the per gallon gas tax.  If the goal is to simply raise revenue regardless of the incentive to reduce externalities, we may see the driving tax.