Senators writing a six-year surface transportation bill are planning to keep status quo spending levels and skip an administration proposal to boost public transit programs and update the nation’s aging infrastructure.

That means lawmakers could miss another chance to prepare the nation’s transit infrastructure to deal with a surge in urbanization in the coming decades. Most of the expected U.S. population growth of 100 million new residents by 2050 will take place in urban areas, making modernization of aging transit systems and construction of new ones more urgent for many lawmakers.

The new authorization measure is expected to be unveiled this week. It’s likely to extend current spending levels for roads, bridges and transit, with some increases to account for inflation. But given that it’s an election year, negotiations are unlikely to produce a long-term fix for the ailing Highway Trust Fund.

And for lawmakers in more rural districts, which depend more on automotive transportation, it’s hard to win support for huge new transit expenses in densely populated urban areas.

The White House had hoped to move its own four-year, $302 billion surface transportation proposal that was sent to Congress as draft legislation last week.

Transportation Secretary Anthony Foxx defended the 62 percent increase the administration wants for transit spending — up to $13.6 billion in fiscal 2015 from $8.6 billion this year — as a necessary shift as Americans increasingly move to urban areas and drive less.

“Many of these communities are trying to get ahead of these population surges by creating transportation options for the future,” he told Senate appropriators.

But the proposal won’t advance amid partisan tensions and election-year jockeying.

Money for Transit

Congress first set up guaranteed formula funding for transit systems as part of a deal brokered in the early 1980s with President Ronald Reagan. It set aside 20 percent of revenues from the Highway Trust Fund for transit, on the premise that boosting transit options for commuters would help reduce congestion on the interstate and national highway systems, producing a direct benefit for fuel tax payers who generate the bulk of the trust fund’s revenue.

The American Public Transportation Association, which represents transit agencies from around the country, contends that the Obama administration’s overtures on boosting transit spending will go a long way toward building out transit systems that have, in many cities, experienced a crush of riders as fuel prices remain elevated, and younger workers choose to commute less in their cars.

In 2013, American transit systems accounted for 10.7 billion trips, the highest numbers in 57 years, said Michael Melaniphy, the transit group’s president and chief executive officer.

“Clearly, people want better transportation options and it should be a national priority to expand public transit service,” Melaniphy said.

Getting new transit projects off the ground hasn’t been an easy lift for states and cities that want them. Spending austerity across all local, state and federal governments have only exacerbated the transit budget crunch.

New York’s Metropolitan Transportation Authority, which operates the subway, bus, ferry and commuter rail systems that serve the nation’s largest city, has been struggling for years to cobble together enough money to build a new subway line beneath Second Avenue on Manhattan’s east side, one of the most dense urban areas in the world.

It would be the first such project since the 1940s, but its long-term viability isn’t yet assured: A two-mile, three-station Phase One is expected to open in the next two years.

But the remainder of the 8.5-mile project, stretching down into lower Manhattan with another 13 underground stations, still doesn’t have firm funding sources for its estimated $17 billion total cost, which clouds the timeline of when its later phases might be completed.

Other states, such as Virginia, have increasingly looked to alternative financing to pay for expensive transit projects.

In the Old Dominion’s sprawling northern suburbs, lawmakers turned to the federal Transportation Department’s Transportation Infrastructure Finance and Innovation Act loan program to help finance construction of the final phase of a new Silver Line that will serve the far-flung Dulles International Airport and surrounding neighborhoods, about 25 miles west of downtown Washington.

Rep. James P. Moran, D-Va., who represents a swath of suburbs that will be served by the new line, said building the new line was a sensible use of tolls collected on the adjacent expressway that serves the airport, which will gradually pay back the federal loan.

“Paving our way out of the worst traffic in the nation is impossible,” Moran said. “This extension is critical to reducing congestion, commuting time and gas consumption.”

The need to diversify federal spending on modes other than roads isn’t lost on Republicans, either. House Transportation and Infrastructure Chairman Bill Shuster, R-Pa., hasn’t championed transit spending with the same enthusiasm as some of his Democratic counterparts. But he has recognized that simply continuing to expand roadways won’t alleviate congestion that plagues densely developed areas in the Northeast, where he has suggested drawing investment to intercity rail would help reduce the need for new lanes on I-95, the traffic-choked road that connects Washington to Baltimore, Philadelphia, New York and Boston.

Transit Shift

The latest data on Americans’ driving habits suggests more transit spending would be welcome.

The Federal Highway Administration in February reported that total vehicle miles traveled — a tally of all the miles driven by all vehicles in the country — in 2013 increased just less than 1 percent, while that tally, when divided by the country’s population, actually dropped for the ninth year in a row.

By that count, there was a decline from 9,402 miles per capita driven in 2013, down from 9,412 miles per capita driven in 2012.

But not everyone is in favor of expanding spending beyond roads.

The American Trucking Associations, led by former Kansas Gov. Bill Graves, a Republican, viewed the White House’s call for increased transit spending as a threat to more robust investments in roads and bridges that carry truckers and their cargoes.

“This administration needs to make much-needed investments in repairing our existing roads and bridges and looking for ways to add capacity to meet our growing needs,” the group’s chairman, Phil Byrd, said late last month.

Graves and other trucking industry officials note that total driving has still ticked up even as personal driving has decreased, meaning much of the pickup in recent months is due to increased commercial traffic as the economy continues to rebound.

Some rural lawmakers have criticized the administration’s plan to put more money into transit but not boost road spending at a similar level. Sen. Susan Collins of Maine, the ranking Republican on the Transportation-HUD Appropriations Subcommittee, lamented that the administration’s proposed increase in transit spending wasn’t helpful for her mostly rural state, which is in need of investment for existing roads and bridges.

Similarly, the conservative Heritage Foundation has been long critical of efforts to expand non-highway spending at the federal level, reasoning that deciding matters of regional transit mobility should be left to states and cities.

“Construction of the interstate highway system was largely complete several decades ago, but Congress has prolonged the federal highway program by repeatedly expanding its scope, inserting itself into purely state and local matters,” Heritage analyst Emily J. Goff wrote in a recent critique of lawmaker efforts to generate a new highway bill.

Joshua L. Schank , the president and chief executive officer of the Eno Center for Transportation, said such regional disagreements could be settled to transit backers’ satisfaction if federal funding formulas were looser, allowing states like Maine to spend more of their money on roads and more dense states to focus on transit.

“If the local needs focus around transit, the ability to be flexible in funding would certainly go a long way toward solving those needs,” Schank said.