Nearly everyone agrees that it’s critical for the country to improve its transportation infrastructure. Doing so will create jobs in the short term and, perhaps more important, help strengthen the backbone of the economy that allows workers to commute to their jobs and businesses to ship goods across the country. As President Obama said in his State of the Union speech earlier this year, “There’s never been a better time to build.”

Here are 10 of the nation’s highest-profile infrastructure projects -- five making steady progress, and five facing some serious challenges -- under way right now.

The Five That Are Happening

1. Dulles Transit Extension

Washington, D.C., residents often try to avoid flights out of Dulles International Airport, located more than 20 miles from downtown, for good reason. With no Metro rail access, trekking to Dulles often involves a long car ride, an infrequent bus or an expensive taxi. That will soon change, thanks to the biggest expansion in the history of the Metro system. Upon completion, the Silver Line will provide public transportation access from downtown D.C. to Dulles, as well as the Northern Virginia suburbs of Tysons Corner, Herndon, Reston and Ashburn. In addition to connecting the airport to the region’s transit system, the new line should help reduce traffic congestion on the Beltway.

Construction began on the long-awaited expansion in 2008. The new line, almost all of which is above ground, consists of 11 new stations and 23 miles of new track. Who’s paying the estimated $6.2 billion cost of the project? The federal government is footing $975 million, and the state is kicking in $200 million. The rest will be funded by the airport itself, local county governments and through fee increases on the existing toll road leading to the airport. The first phase of the line, connecting D.C. to Reston, is scheduled to open in 2013. If all goes according to plan, the second phase, extending the Metro to Dulles and outlying Loudon County, will be finished in 2016.

2. Otay Mesa East

In the San Diego region, there are two border crossings. One is the busiest U.S.-Mexico crossing for personal travelers. The other is the second-busiest crossing for commercial trucks. As a result, vehicles traveling north can find themselves sitting in line for hours to cross. Local economic studies argue that those delays are costing the U.S. billions of dollars each year, with much of that loss concentrated in the San Diego area.

To alleviate the bottleneck, state and local officials are planning a third land port in the area. The proposed Otay Mesa East port facility, along with a new 2.5-mile roadway connecting it to the highway system, could cost as much as $715 million. Officials are banking that speedier travel times could be an economic boon for the region, since it would make visits to San Diego easier and more efficient for both Mexican tourists and for shipping companies. They’re also betting that visitors are willing to pay tolls to avoid the congestion at the other facilities. Consultants are currently studying just how much traffic the new crossing could expect and how much drivers would be willing to pay to bypass congestion at the other crossings.

The federal government authorized a new border crossing in 2008, but what’s more crucial is getting the commitment from U.S. Customs and Border Protection that it will staff and operate the facility. That makes the study critical, since that data will help show just how much pent-up demand exists for a new facility. While the region can build the road that connects to the crossing -- construction could start late this year or early next year -- it will have to wait on the port-of-entry until getting the go-ahead from Washington, says Marney Cox, an economist with the San Diego Association of Governments. “We clearly need the feds to give the thumbs up on that one.”

3. O’Hare Modernization

More than 65 million passengers pass through Chicago’s O’Hare International Airport each year, and those travelers have experienced their fair share of delays. In fact, according to the U.S. Department of Transportation, almost a quarter of the flights both in and out of the nation’s second busiest airport weren’t on time. As O’Hare traffic grew during the 1970s and 1980s, the airport’s capacity couldn’t keep up. But now a major modernization project is increasing the facility’s efficiency and capacity, helping to reduce flight delays. In 2008, O’Hare finished a new runway, extended another and built a new air traffic control tower. The Chicago Department of Aviation hopes to complete two more runways in 2013 and 2016.

The modernization effort -- estimated at $8.8 billion -- is being paid for with a mix of airport revenue bonds, grants from the Federal Aviation Administration (FAA) and passenger fees. According to a recent report from the Eno Center for Transportation Policy, a national think tank, Chicago “has managed to finish key aspects of the work under budget and ahead of schedule.”

Transportation expert Joseph Schwieterman, a professor at Chicago’s DePaul University, says he agrees the effort has been well run. “The biggest winners,” he says, “are the region’s economy and O’Hare fliers, who will see better service and lower fares.”

4. Crescent Corridor Expansion

The existing system for moving most goods across the country has long been based on railroads and an array of interstate highways. But increasing congestion on the nation’s roads has sparked renewed interest in intermodal transportation and an increasing role for methods other than trucking. Spurred by interest from freight companies eager to save money on costly long-haul truck routes, railway company Norfolk Southern is working to upgrade the Crescent Corridor, a freight rail network that runs through 13 states and connects New Orleans to New Jersey. A series of projects will lay 300 new miles of track and build or expand intermodal terminals in 11 markets. Construction has already begun on terminals in Memphis, Birmingham, Ala., and two Pennsylvania communities.

The estimated $2.5 billion expansion will enable shippers and trucking companies to move more of their goods using rail, instead of relying on trucks, which are more expensive and produce more pollution. Because of the public benefits of the cleaner air and reduced highway congestion that will result from the upgrade, the U.S. Department of Transportation has chipped in more than $136 million to support the project, with the rest of the funds coming from states and Norfolk Southern itself. The company hopes to finish the upgrade by 2030 but says the timing depends on how easily it can obtain enough public funds for the project.

5. Alaskan Way Viaduct

In 2001, seismic experts began considering the possibility of strengthening Seattle’s iconic Alaskan Way Viaduct, a double-decker elevated highway that skirted downtown along Puget Sound. The team’s work was prescient: An earthquake struck in the midst of their study, damaging the structure. Replacing the viaduct turned out to be more cost-effective than repairing it. The effort hasn’t been easy. For nearly a decade, the project has been subject to bitter political debate over whether the elevated road, which effectively cut off downtown Seattle from the waterfront, should be replaced with a tunnel. Mayor Michael McGinn won election in 2009 after campaigning against the tunnel, which he said would be too expensive. Since then, however, the tide has turned. In a 2011 referendum, voters endorsed the tunnel plan, allowing the project to move forward.

The south half of the viaduct was demolished in October, and traffic was shifted onto a new bridge near the city’s sports stadiums. A new roadway on the south end is scheduled for completion next year. Meanwhile, the remaining portion of the viaduct -- the section along the downtown waterfront -- will be removed after the new tunnel opens to traffic in late 2015. Boring of that tunnel is expected to start next year. Both the new roadway and the tunnel are designed to withstand an earthquake registering a 9.0 magnitude.

The $3.1 billion project is being funded by a combination of state gas tax revenue, federal funds, tolls and a contribution from the Port of Seattle. Officials with the state transportation department have downplayed comparisons to Boston’s infamous Big Dig, noting that the Seattle tunnel is shorter than Boston’s and seeks to minimize the impact on its waterfront and downtown. A panel reported to Washington Gov. Chris Gregoire earlier this year that the project is on time, on budget and “has the ability to be successfully completed.”

... And The Five in Limbo

1. Columbia River Crossing

This joint Oregon-Washington project, which could cost up to $3.5 billion, would replace the existing Columbia River bridge that connects Portland to the suburb of Vancouver, Wash. Additionally, the project would rebuild a series of highway interchanges around the bridge, extend light rail across the river, improve the existing highway, and build pedestrian and bike paths. Officials say the effort will also improve safety, since there is an average of one collision per day in the project area, nearly double the rate on similar urban highways. It’s viewed as an important investment in the region’s economy, since the more than $40 billion of freight crossing the existing bridge each year is increasingly facing delays due to congestion.

The next 18 months will be critical. That’s when both states’ legislatures will hammer out many of the details of financing the project, which includes a tolling component. Officials hope to begin construction next year. But already, some are starting to balk at the cost. A report last year from the Oregon state treasurer suggested the project may have overestimated its revenue projections from tolling by as much as $598 million. As a response, the project team has put together a scenario that would postpone about $140 million worth of improvements to the Oregon interchanges. Meanwhile, the project relies on $1.25 billion in federal funds, which are by no means assured, given Congress’ focus on spending reductions. If that money doesn’t come through, it “may require rethinking of the overall project scope, timeline and financing plan,” according to the treasurer’s report.

2. Denver FasTracks

One of the country’s most ambitious transit projects, the FasTracks system will culminate in 122 new miles of commuter and light rail and 18 miles of bus rapid transit service across the Denver area. It’s part of the region’s goal to push transit-oriented development that will reduce sprawl, congestion and pollution. Residents voted in 2004 for a 0.4 percent sales tax to fund the $4.7 billion effort. Then the costs started ballooning. The latest estimate is that the project will cost $7.8 billion to complete by the original 2024 target date. In the last year alone, the cost of one particular commuter line has increased from $894.6 million to $1.7 billion, due largely to stalled negotiations with an existing freight line. Some officials are now discussing whether they could cut costs by turning the rail line into a bus route, even though that’s not what voters originally agreed to.

What else went wrong? Thanks to the recession, sales tax revenues didn’t grow as much as expected. (Some critics say the growth estimates were overly optimistic to begin with.) Meanwhile, the costs of materials have increased and the project has been pegged with expensive new safety requirements. “It all just worked together at exactly the right time to scuttle the viability of the project as originally priced,” says Diane Barrett, chief projects officer for Denver Mayor Michael Hancock.

This year, voters will likely be asked to double the tax they’ve allotted to the project. If they fail to do so, the project won’t be fully complete until 2042, according to the Regional Transportation District of Denver. Getting approval from voters is far from a sure thing. Last year officials opted against holding a vote, largely because citizens were expected to reject it. Still, the project is marking some successes. The first of the new light rail lines is on track to open next year, and the renovated Union Station transit hub -- the public face of the project -- is more than halfway complete and expected to open in 2014.

3. NextGen

When airplanes are delayed, nobody wins. Airlines lose money. Passengers become inconvenienced. Airports get overwhelmed. That’s why the FAA is touting an effort that it says could reduce delays by 35 percent by 2018.

The project, which aviation administrators began planning in 2003, is dubbed NextGen, and proponents say it would revolutionize air travel in this country by switching from radar-based to satellite-based flight-tracking technology. That, along with other technological advances like improved weather forecasting and communication systems, would allow planes to fly more direct routes instead of following the existing, inefficient flight paths that are arranged like highways in the sky. The result: More flights in the air at any given time, fewer delays and less wasted fuel.

But the cost is enormous. FAA officials say they’ll need between $20 billion and $27 billion for the project through 2025. The Government Accountability Office says the cost could actually be as high as $160 billion. Meanwhile, there’s an ongoing debate about what proportion of the cost should be picked up by the airline industry, which has historically been skeptical of the benefits of government-mandated technologies. A recent report from the Department of Transportation’s inspector general said the system will likely face delays because the “FAA has not made critical, longer-term design decisions on NextGen ground and aircraft systems.”

To complicate matters, the FAA has spent more than four years without a long-term funding bill, thanks to congressional inaction. That’s made it difficult to pursue larger projects like this one. A long-term bill signed earlier this year should help on that front, but the funding for the effort is still in question. The president’s 2013 budget calls for just over $1 billion for NextGen, which is a drop in the bucket. In a Congress focused on spending cuts, launching something like NextGen could be tough. “I’m guessing we’ll muddle along,” says David Plavin, an aviation consultant. “They won’t provide the big, incremental investment … that’s ultimately necessary.”

4. California High-Speed Rail

It started off with the kind of heady promise and excitement that prompted comparisons to California’s most iconic infrastructure project, the Golden Gate Bridge.

The visionary plan was for 800 miles of high-speed rail lines connecting Los Angeles and San Francisco. Riders could whoosh from Southern California to Frisco in an unheard-of two hours and 40 minutes. The train would reduce air pollution and ease congestion on the state’s famously clogged freeways. Construction would create tens of thousands of new jobs. Voters in 2008 approved $9.95 billion in bonds to usher in a new era of transit for the Golden State.

But times have changed, and the past year has been a rough one for the project. A recent poll showed that 59 percent of Californians would vote against the bond if they could do it again. Cost estimates have grown from $43 billion to at least $98 billion, and the completion date of the first phase has been pushed back 13 years. The project has been subject to congressional hearings. Both the CEO and the project chairman resigned. Most recently, a state auditor’s report noted that the largest potential funding source for the project is the federal government, and it’s unclear what happens to the project if those funds aren’t secured.

Gov. Jerry Brown has staked his reputation on the project, and its new board chairman is a longtime aide of his. Brown chose to highlight high-speed rail in his State of the State speech in January. He’s hinted that a finalized business plan will bring its costs below $98 billion, and he says he’s working to find new revenue sources.

5. Second Avenue Subway

First proposed in 1929 and more recently dubbed by New York magazine as “the line that time forgot,” the Second Avenue subway line on the East Side of Manhattan has been a perennial wish-list item for the better part of a century. The line would ease overcrowding on the Lexington Avenue line, which currently shuttles 1.3 million people every day, but over the years it’s been repeatedly delayed by economic downturns and political in-fighting. A construction attempt in the 1970s was eventually aborted due to lack of funds. Finally, in April 2007, the Metropolitan Transportation Authority (MTA) broke ground on the Second Avenue line. The first phase -- a $4.45 billion two-mile section with two new tunnels and three new stations on the Upper East Side -- could be operational by the end of 2016, according to the MTA.

But the Federal Transit Administration is not as optimistic. It believes the project could take significantly longer and will end up costing $410 million more than MTA’s estimate. And that’s just Phase 1. Extending the line uptown to Harlem and downtown to the Financial District, a total of 8.5 miles, is expected to cost at least another $13 billion, and none of that money has been secured. A fully functional Second Avenue line could remain a Big Apple fable for many years to come.