The federal Highway Trust Fund, which represents about one-quarter of total public spending on transportation infrastructure and transit projects across the country, is projected to run out of money by Aug. 29, according to the latest update from the federal Department of Transportation.

The highway portion of the fund is projected to reach a critically low level of $4 billion around July 25, at which point payments will not be guaranteed in a timely manner.

Federal gas tax revenue goes to the Highway Trust Fund, but this revenue source has declined in recent years due to decreased fuel consumption. In Congress, House leaders do not favor increasing the gas tax in order to increase revenue.

The federal gas tax of 18.4 cents per gallon hasn’t been raised since 1993. If it were indexed to inflation, the tax would be approximately 30 cents per gallon now.

The federal gas tax brings in about $34 billion a year, but transportation projects slated for the current year would require about $50 billion in funding.

In Massachusetts, several large projects would stall if a funding solution is not identified and implemented. According to the Boston Globe, these include the reconstruction of Interstate 91 in Springfield, the resurfacing of Route 1 in Peabody, and bridge replacements on Route 16 in Medford and Everett. Transit projects such as the extension of the MBTA Green Line and expansion of Boston’s South Station also would be jeopardized.

Gov. Deval Patrick, quoted in the Springfield Republican, said a failure to authorize spending from the federal trust fund could slow or stall up to half of all planned bridge projects in Massachusetts. He also noted that work on I-91 in Springfield must begin soon or the highway could be forced to close.

The advocacy group Transportation for America reports that no new transportation projects will be funded in the next federal fiscal year, which begins on Oct. 1, unless the Highway Trust Fund is replenished.

Highway infrastructure and transit projects funded through this revenue stream employ approximately 700,000 people across the country.

In addition to the Highway Trust Fund shortage, Congress must contend with the Sept. 30 expiration of the current two-year surface transportation bill, Moving Ahead for Progress in the 21st Century, or Map-21. In late April, President Barack Obama filed a four-year, $302 billion surface transportation reauthorization bill, called the GROW AMERICA Act. In May, the Senate released a six-year, $265 billion surface transportation bill. The House has not yet released a proposal.

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