Unless Congress extends it, the 18.4 cents-a-gallon federal gas tax will expire on Sept. 30. Allowing that to happen would be tremendously destructive. It would bankrupt the already stressed Highway Trust Fund, with devastating effects on the country’s highways, bridges, mass transit systems and the economy as a whole.

Reports suggest that some House Republicans may push to let the tax lapse or use the threat of expiration as leverage in the budget wars. This is a dangerous idea. If anything, the tax should rise to maintain a system that constantly needs upkeep — the backlog of bridges needing repair is estimated at $72 billion — to create jobs, and to encourage drivers to buy more fuel-efficient cars.

Excise taxes on motor fuels account for nearly nine-tenths of the $37 billion trust fund. The fund has lately required annual infusions from the Treasury Department to break even, and its obligations are growing. The gas tax has not increased since 1993, and its buying power, accounting for inflation, is now only 11 cents. Meanwhile, Americans are driving many more miles, placing greater stresses on the highway system.

When state taxes are added in, Americans pay, on average, about 43 cents per gallon in taxes — or about one-eighth the total price at the pump. That’s still a bargain compared with other industrial countries. Across Europe, drivers pay twice what Americans do at the pump — and well over half of that is taxes. In Britain, the tax bite is more than $4 a gallon, or 10 times what Americans pay.