Why is it so damn hard to raise the gasoline excise tax?

See "Drop in Miles Driven Is Depleting Highway Fund; Loan From Mass Transit [Account] Is Urged," from the New York Times. From the article:

Gasoline tax revenue is falling so fast that the federal government may not be able to meet its commitments to states for road projects already under way, the secretary of transportation said Monday.

The secretary, Mary E. Peters, said the short-term solution would be for the Highway Trust Fund’s highway account to borrow money from the fund’s mass transit account, a step that would balance the accounts as highway travel declines and use of mass transit increases. Both trends are being driven by the high price of gasoline and diesel fuel. ...

That dampens proceeds from the federal tax on motor fuel: 18.4 cents a gallon on gasoline, 24.4 cents a gallon on diesel. Revenue from the tax goes to the Highway Trust Fund, with most of it designated to the highway account, which finances construction and repair of roads, and a much smaller share to the mass transit account.

Also see "The Changing Commute - Demand for Rides Soars" from the New York Times, "Riders flock to T in record numbers" from the Boston Globe, and remember these four press releases from July from the DC region's transit authority:

• Metrorail records third high ridership day of the week
• Metrorail sets back-to-back high ridership days
• Metrorail sets ninth highest ridership day
• Metrorail sets new record for highest ridership day of all time

Note that while the typical driver believes that gasoline excise taxes (state, federal, local + registration fees) fund roads to the tune of 100%, the actual amount is closer to 50%. The rest comes from general funds and other sources.