Fairfield County has the fourth-highest number of motorists using structurally deficient bridges among metropolitan areas nationwide with populations of 1 million or fewer, according to a new analysis by a Washington, D.C., transportation advocacy group measuring daily traffic on aging and deficient bridges.

"The Fix We're In For: The State of Our Nation's Busiest Bridges," said Fairfield County averaged 1.38 million drivers daily using one of 107 spans in the Bridgeport/Stamford/Norwalk area the Federal Highway Administration categorizes as structurally deficient.

The numbers equal one Fairfield County driver crossing a structurally deficient bridge once every 16 seconds, the report states.

Steven Higashide, a federal advocate for the Tri-State Transportation Campaign, said the numbers in Fairfield County and other regions should be of major concern, given a lack of commitment so far in Congress to approve a long-range transportation funding bill with dedicated funds to address a growing number of deficient bridges.

"With federal funding involved in more than half of the projects in the state of Connecticut, it is difficult to see how the state could maintain the bridges without more investment," Higashide said.

The New Haven/Milford area was ranked sixth among urban areas with populations of 500,000 to 1 million, with 1.32 million passengers using 56 bridges categorized as structurally deficient.

Connecticut ranks 20th among 50 states and Puerto Rico in its total percentage of deficient bridges, according to a report issued by Transportation For America in April.

While the increasing average age of spans is a concern for the state Department of Transportation, spokesman Kevin Nursick said the structurally deficient category doesn't mean bridges are unsafe for drivers, but that a repair is required to reduce load capacity called for in the bridge's initial design.

Connecticut bridge inspectors now conduct more than 5,000 inspections every two years, he said.

"We take the business of bridge inspection very, very seriously, and there is no way we would allow the public to travel on bridges that are not safe," Nursick said. "It is just not going to happen."

The DOT maintains about 4,000 bridges in all, with just under 3,000 of them 20 feet or longer, of which 189 are structurally deficient, Nursick said.

Of the more than 1,000 bridges 20 feet or shorter the DOT cares for and inspects, 119 are structurally deficient, he said.

Nursick said Connecticut transportation officials have been unable to firmly set their long-range capital plans for bridge repairs and other projects because of uncertainty over when and what kind of surface transportation reauthorization bill Congress might pass and if it would boost Connecticut's funding over a series of several years.

"It's been awfully difficult to plan five years out, or 10 years out, which you want to do when you don't have a consistent understanding of what your funding level is," Nursick said. "We haven't seen any increase and been running on continuing resolutions, so most DOTs don't know what they are going to get."

Since the previous version of the bill expired in 2009, Connecticut and other state DOTs have been receiving transportation funds from the federal government through interim appropriations.

Over the next five years, Connecticut is expected to need about $2.4 billion to finish maintenance and preservation projects required to maintain transportation infrastructure in good repair, including some highways and bridges built in the 1950s and 1960s, according to a January report written by a working group that served as part of Gov. Dannel P. Malloy's transition team.

"We are holding the line on maintaining our structures and overall in comparison to other states and particularly structures as old as we have," Nursick said. "But because bridge work is so expensive, we are going to need continuing support and, optimally, higher funding levels from our federal partners to maintain this aging infrastructure."

Amanda Kennedy, associate planner for the Regional Plan Association, a member of the Transportation for America coalition, said the report highlights the need for higher levels of federal funding for bridge work nationwide and for Connecticut to remain focused on fixing existing bridges and roads rather than beginning new construction projects.

"It really is the traffic volume and the age of the bridges, and we're starting to really face the fact that a lot of these aging bridges need major work or replacement," she said.

Kennedy said inevitably within the next decade, the call to overhaul infrastructure nationwide will require new funding mechanisms, such as tolling on some roads or a vehicle mileage tax on drivers.

"We need to have transit alternatives out there to get people to work without burdening our existing road system, and then we need to find a way to raise the revenue," Kennedy said. "... It's not a conversation that is going to go away."