Similar traffic declines have occurred around the region, challenging long-established assumptions about the need for bigger highways and bridges.
"If these trends continue, it would definitely change the way we need to plan for our transportation future," said Chris Puchalsky, associate director of systems planning at the Delaware Valley Regional Planning Commission. "But I think the jury is still out on that . . . we need two or three more years of data."
In 2007, the Pennsylvania Turnpike Commission assumed that traffic would grow 3 percent to 5 percent every year to help pay for debt as it took on a new obligation to contribute up to $900 million a year to fix other roads around the state.
Instead, traffic has been essentially flat.
And when the Delaware River Joint Toll Bridge Commission decided in 2003 to replace the 50-year-old, four-lane Scudder Falls Bridge on I-95 with a $328 million, nine-lane, 180-foot-wide toll bridge, it assumed that traffic would increase 35 percent by 2030.
In fact, bridge traffic has declined slightly and is now below the levels of 2002.
Drops also have occurred over the last six years on the Schuylkill Expressway and Interstates 95 and 476, according to data compiled by the Delaware Valley Regional Planning Commission.
Highway planners misjudged the future because the Great Recession reduced both commercial and passenger travel, and because of an unexpected drop in driving by young adults.
Now, planners and policymakers must decide whether the last decade was an aberration or the beginning of a new normal.
The decisions are taking on new urgency, as Congress struggles to come up with a new transportation-funding plan by the end of September, when the current one expires. The federal Highway Trust Fund, which pays for road projects around the country, is nearly broke.
"The last decade was a really tough decade for forecasting," said James W. Hughes, dean of the Edward J. Bloustein School of Planning and Public Policy at Rutgers University.
Traditional expectations of economic growth - which typically fuel traffic growth - were undone by the recession of 2001, the Great Recession of 2007-2009, and anemic job growth for the entire decade, Hughes said.
Add to that the unprecedented behavior of young adults, driven by technology, lifestyle choices, and economic prospects.
"The millennials are really changing the world dramatically," Hughes said. "We have a younger generation that is driving less and doesn't want to live in Valley Forge. They want to live in Center City Philadelphia."
"We had a 50-year period of unrestricted suburbanization, and now there's a dramatic shift."
Cars and driving are less important to young adults, who find that trains and buses allow them to work and socialize on mobile electronic devices, he said.
That may mean fewer cars on future roads.
"Nobody was really anticipating this," Hughes said. "The models have to be recalibrated."
Some projections have already been lowered.
Traffic predictions for the planned new Scudder Falls Bridge over the Delaware River between Bucks and Mercer Counties were revised downward in 2010, said Joseph Donnelly, spokesman for the Delaware River Joint Toll Bridge Commission. And yet another traffic study, costing $452,128, was ordered in October, to get a better handle on future traffic and revenue.
But Donnelly said a new bridge was needed, regardless of how projections may change, because "the bridge has trouble handling the traffic it has now."
Likewise on the $2.5 billion widening of the New Jersey Turnpike between Exits 6 and 9, according to Turnpike Authority spokesman Thomas Feeney.
"Even if traffic volumes on the turnpike remained flat forever, the widening would have been necessary," Feeney said. "That widening area is one of the worst bottlenecks on any highway in New Jersey. . . . The additional capacity is needed today, and the benefits are going to be immediately apparent to drivers as soon as the project is completed this November."
For the Pennsylvania Turnpike, anticipated traffic growth - along with annual toll hikes - was crucial to the turnpike's plan for paying up to $900 million a year for statewide highway and transit projects.
A 2007 state law required the Turnpike Commission to borrow billions of dollars to make those statewide payments, with the expectation that the debt would be repaid by converting Interstate 80 into a toll road and by raising tolls on an ever-increasing number of vehicles on the turnpike.
Two of those things didn't happen.
The federal Transportation Department rejected Pennsylvania's bid to toll I-80, and traffic did not increase on the turnpike.
The Turnpike Commission's annual financial obligation for statewide projects was cut to $450 million in 2011, but its debt continues to rise. It is now more than $8 billion in the red.
Nikolaus Grieshaber, the turnpike's chief financial officer, said traffic-growth projections have been cut in half for the future, down to 1.5 percent a year through 2041.
The more-cautious projections take into account the effect of annual toll increases, as well as the less-robust economy, Grieshaber said. Those decisions limit how much money the commission can expect to spend on maintenance and construction.
"We're trying to analyze what's going on," he said, "and how we may need to address our projects going forward."