(Gerald Marineau/The Washington Post) (Gerald Marineau/The Washington Post)

The Federal Highway Trust Fund, which uses the 18.4 cent federal gas tax to pay for roads and bridges, will run out of money in the third week of August, the U.S. Department of Transportation projected on Tuesday.

USDOT began spotlighting the looming funding crisis several months ago. The federal agency hoped to get more attention from average Americans, many of whom seem unaware that a source their state transportation agencies have relied on for decades is about to dry up.

The gas tax has not been increased since 1993, and the Highway Trust Fund, into which the revenue flows, has suffered because the tax has not kept pace with inflation. Plus, improvements in vehicle fuel economy have reduced consumption.

The trust fund began the fiscal year with $1.6 billion. A few weeks later, $9.7 billion was transferred into it from the general fund. (Congress had authorized a $10.4 billion transfer, but that was reduced under sequestration.)

But spending on what’s known as surface transportation has continued at a greater pace than that of money coming in, and the construction season that uses lots of cash begins in just a few months.

USDOT said that as of the last week of January, the trust funds had $8.3 billion in cash.