Parts of Interstate 95 south of Fredericksburg may become toll roads if Virginia follows through on a federal approval of a plan to pay for highway improvements by collecting money from drivers who use them.

Preliminary approval by the Federal Highway Administration (FHWA), which has authority over tollling on interstates, allows the state to move ahead with a proposal to require tolls from drivers heading into the state from North Carolina.

Although there has been strong resistance in Congress and in many states to replace waning gas tax revenue with pay-by-the-mile plans, the Virginia proposal reflects a stark reality faced by state officials nationwide. They don’t have enough money to maintain and replace an aging network of roads and bridges.

“This approval is a major step toward funding critical capacity and infrastructure improvements needed in this corridor,” Virginia Gov. Robert F. McDonnell said Monday in a statement announcing the approval. “The General Assembly passed my transportation plan setting the framework for investing $4 billion in our transportation network over the next three years. The ability to toll I-95 will help leverage this investment by funding transportation improvements in this vital corridor.”

The state estimates it would collect $250 million in tolls in the first five years of operation and more than $50 million a year after that. Under federal rules, toll money would have to be used to improve or expand capacity on the highway where it is collected.

To win final federal approval of the I-95 tolling plan, the state must conduct an environmental review, outline improvements that will increase capacity, determine where tolls will be collected and defend that determination, and certify that toll revenue will replace all other federal funding that otherwise would have been used for maintenance or improvement of the tolled portions.

“Our goal is to complete these steps as quickly as possible so we can develop and implement a satisfactory toll agreement with the FHWA,” said Virginia Department of Transportation Commissioner Gregory A. Whirley.

If the state meets the requirements, Virginia officials have estimated it would take 18 to 24 months before toll collection would begin.

“The entire I-95 corridor averages a level of service of ‘D,’ and some more urban portions are ‘F’ during peak periods,” said Virginia Secretary of Transportation Sean T. Connaughton. “This level of service is unacceptable anywhere, let alone on the most traveled corridor in Virginia. The ability to implement tolling will provide the revenues necessary to improve I-95.”

The federal Highway Trust Fund, the principal source of transportation funding, no longer can keep pace with the nation’s needs. Its money comes from the 18.4 cent-per-gallon federal gas tax, which has not been raised in 18 years even as the average gas mileage of cars in the United States has increased significantly.

Inability to resolve that revenue gap is the primary reason that Congress has been unable to pass a long-term transportation spending plan since the last one expired in 2009.

The gap is enormous: The U.S. Chamber of Commerce estimated six years ago that $222 billion a year was needed to maintain the surface transportation system. Revenue flowing into the Highway Trust Fund was falling about $45 billion short of that amount.

A congressional commission in 2009 considered more than two dozen options before recommending a national transition from a fuel-tax-based revenue system to one “measured by miles driven.”

Though neither chamber has formally introduced a surface transportation bill, leaders in the House and Senate have outlined strikingly different proposals.

The Senate proposal would provide about $109 billion spread over two years, an outlay that would require general-fund money to augment trust-fund revenue.

The House, which is committed to a spending plan based on gas tax revenue, has talked of a six-year plan to provide about $35 billion a year.

House Transportation Committee Chairman John L. Mica (R-Fla.) thinks that sum can be used to leverage double that amount through public-private partnerships. When those partnerships take the form of arrangements such as the high-occupancy toll (HOT) lanes under construction on the Capital Beltway in Virginia, the private partner profits by charging tolls.