The April 30 news article “White House proposes letting states charge tolls on interstates” offered an interesting look at a range of options for funding improvements in our transportation infrastructure. But the article misdiagnosed the cause of the looming shortfall in the Highway Trust Fund when it stated that the fund “relies on the 18.4-cent federal gas tax, which has eroded steadily as vehicles have become more energy efficient.”

The major reason for the shortfall is that Congress failed to plan for growth in the cost of building and maintaining roads. This October, the 18.4-cent gas-tax rate will turn 21 years old, but 18.4 cents cannot buy nearly as much asphalt and machinery as it could two decades ago.

It is true that fuel efficiency has chipped away at the gas-tax base by allowing drivers to travel farther on each tank of gas. This will become an increasingly important issue if fuel efficiency improves as expected. But this impact has paled in comparison with the substantial erosion brought about by inflation.

Carl Davis, Washington

The writer is a senior analyst at the Institute on Taxation and Economic Policy.