A partial bridge collapse over the Skagit River in Washington in May prompted public reflection in communities across the nation. People naturally want to know whether they can trust that bridge they cross every day on their morning commute.

We spent several weeks talking to bridge inspectors, experts and state officials to get a better sense of how our own area bridges are rated. We also pored over data to put together a geographic snapshot of bridges in Southern California. What we discovered is that monitoring the health and safety of our infrastructure is a complicated matter — far too complicated to boil down to a simple catch phrase.

News reports abound with quick figures and ominous terms that seem to condemn our public bridges as "structurally deficient" or "functionally obsolete." But those terms, along with an often cited federal "sufficiency rating," do not predict whether a bridge is in any danger of collapsing. To be sure, there are bridges in desperate need of repair, but let's be sure we've got the terms straight.

Q. What does "Functionally Obsolete" mean?

Functionally Obsolete is a federal designation that means a bridge isn't up to current building standards. Often that means the lanes are too narrow or there is no shoulder on the side of the bridge for stalled cars. Less frequently it means there could be structural problems serious enough to warrant a "high priority of corrective action." The Federal Highway Administration uses a scoring system to determine whether to label a bridge "functionally obsolete."

Q.What does "Structurally Deficient" mean?

Structurally Deficient is a federal designation that means a bridge has physical flaws of some kind. These could include cracks or weaknesses in the bridge's key support elements or frequent waterway overflows causing minor to severe traffic delays. In most cases they are just cracked pavement or peeling paint. The Federal Highway Administration uses a scoring system to determine whether to label a bridge "structurally deficient."

Q. What does "Fracture Critical" mean?

Fracture Critical is a federal designation that means a bridge doesn't have a secondary support system to stop it from collapsing if it is seriously overloaded or experiences a significant accident that impacts a vulnerable section. Both the bridge in Washington state and the one that fell in Minnesota in 2007 were Fracture Critical. However, a label of "Fracture Critical" does not necessarily mean a bridge is structurally unsound.

Q. What's the difference between the federal "Sufficiency Rating" and California's "Bridge Health Index"?

The state and federal governments each have a system for rating bridges on a scale of 0 to 100, but the criteria for determining those scores vary in important ways.

The Federal Highway Administration's "sufficiency rating" is based on a number of factors, not all of them related directly to a bridge's structural integrity. In fact, only 55 percent of the score comes from the structural adequacy and safety of the bridge. The rest of the score is based on considerations of the bridge's importance to public use and whether it is functionally obsolete.

By FHA standards, a deficient bridge is one that has a sufficiency rating below 80 and is also labeled "functionally obsolete," "structurally deficient" or both.

Transportation officials in California and elsewhere have expressed concern that the federal rating system comes up short when trying to identify and prioritize bridges that need to be fixed or replaced. An alternative measure has been developed in California that relies on a more detailed and methodical inspection of hundreds of individual bridge elements. The Bridge Health Index takes into consideration both the condition and the economic value of each element on a bridge. In the same way a car begins losing value the moment you drive it off the lot, the bolts and girders and other parts of a bridge lose value as they age. The health index takes into consideration the value of each element and combines that information into a single score.

In an article in the journal of the Transportation Research Board of the National Academies in 2001, two Caltrans officials argued that the state's goal should be to have no more than 5 percent of the state's bridges with a health index below 80.

Still, determining a bridge's overall health is a complicated matter and prone to human error. The state considers these ratings and more in monitoring its inventory of bridges, and experts continue to look for ways to improve the science of bridge management.

Q. Why are there so many different terms and ratings systems?

Different systems are used for different purposes. The Federal Highway Administration’s system is used mainly to decide if a bridge qualifies for federal rehabilitation money. Not all bridges that qualify need to be repaired. The Bridge Health Index is used by local officials to help determine which bridges are most in need of repair or replacement.

Q. Why can't the government predict when a bridge is going to fail?

Officials often prevent bridges from falling by repairing or rebuilding them. However, some bridges present unforeseen issues, like the I-35 bridge that collapsed in Minnesota in August 2007. The National Transportation Safety Board believes that a design flaw combined with added weight on the bridge from construction equipment led to the collapse. At the time, the Minnesota state government did not account for these factors when rating the bridge, but it has since changed its inspection practices. Investigators are still looking into why the I-5 bridge in Washington state collapsed in May 2013, but they believe an oversized truck that struck a support beam caused it to fall. In both cases, inspectors had regularly monitored these bridges but had not foreseen these disasters. Some engineers believe the solution is to embed sophisticated sensors in bridges that could constantly monitor and report on the structure’s integrity.

Q. Who's responsible for inspecting bridges?

The California Department of Transportation is responsible for assessing the health of the more than 24,000 bridges owned by the state and local governments. Caltrans then makes recommendations for repair as needed.

Q. What is the general state of health of California’s bridges?

The transportation infrastructure advocacy group Transportation for America ranked California 18th worst in the nation in 2010. It said that 12.8 percent of the state’s bridges were rated “structurally deficient” compared with 11.5 percent nationwide. However, some experts say that California’s bridges may be in better shape than those numbers suggest. UCLA Associate Professor of Structural Engineering Jian Zhang says “a lot of the structural deficiencies have been identified after the past earthquakes and they have been improved.” She also says after the Lome Prieta quake in 1989 and the Northridge quake in 1994, many bridges were retrofitted to be more seismically sound. Still, Zhang and others say that California’s bridges could use more funding for upkeep.

Q. How often do bridges fail in California?

No bridge in California has fallen due to neglect, according to Caltrans officials.

Q. How often are bridges evaluated?

Most bridges are inspected every two years. Some bridges are inspected more frequently if they have problems that Caltrans wants to monitor more closely. Bridges in rural areas with low traffic are inspected once every four years.