The next person who mentions the terms "structurally deficient" or "functionally obsolete" in my hearing will be punched in the mouth.
From talking heads on cable news to engineers and policymakers who really ought to know better, misuse of these terms has been rampant since at least August 2007. Let's set things straight with a healthy dose of facts and the not-so-subtle threat of violence.
Structurally deficient (SD) means that load-carrying elements of the bridge have been rated as "poor" or worse (4 or below on the National Bridge Inspection Standards scale of 0-9, where nine is brand-new and zero means the bridge has fallen into the river) or that the waterway opening provided by the bridge is inadequate1.
Functionally obsolete (FO) means the bridge is inadequate from the perspective of roadway geometry issues1 (traffic capacity, roadway shoulder width, and approach sight lines, for example).
Let's examine the abuse of these terms in detail:
First, and most importantly, SD and FO are planning, not structural engineering, terms. They do not necessarily reflect the safety or serviceability of a bridge. Bridges designated as SD or FO get higher priority for repair and rehabilitation. As a consequence of this, it can actually be good for a county engineer to have a bridge designated SD or FO - state or federal money may then become available.
Structurally deficient is not a synonym for "unsafe" or "near collapse." Recall that the SD designation is based upon a load-carrying element being rated 4 or below on the National Bridge Inspection Standards (NBIS) scale. 4 is basically the lowest rating at which an element can be left in place as-is. You have probably driven over a bridge with an element rated 4 within the last seven days. That doesn't mean your life was in danger.
Reporting the sum of SD and FO bridges is especially meaningless from a bridge safety perspective. I can't tell you how many presentations I've attended where the first words from the speaker's mouth were "Since X percent of the nation's bridges are structurally deficient or functionally obsolete..." - from a safety perspective, this is about as meaningful as saying "X percent of the nation's bridges are painted green." FO has absolutely no structural safety meaning. A bridge can be rated FO because its shoulders are too narrow, or because it has an old-style guardrail. That doesn't mean the bridge is going to fall down. One very common cause of an FO designation is that traffic has increased on the route carried by the bridge. A bridge can be deemed FO even if it has a good structural rating of 7, 8, or 9. So, while FO bridges ought to be upgraded or replaced to maintain good levels of service throughout the network, an FO designation doesn't provide any more indication of a bridge's safety or serviceability than the color of the bridge paint. If you're trying to make a point about structural capacity, don't inflate your numbers by counting FO bridges too.
The federal definition of a "bridge" is any structure with a clear span of at least 20 feet2. That includes everything from the Golden Gate Bridge down to structures that are little more than oversized culverts. The SD and FO criteria may be applied to any structure in this spectrum. When we refer to the number of SD/FO bridges, an outdated bridge on a low-volume county road counts the same as the heavily traveled (and both SD and FO) Milton-Madison (Kentucky/Indiana) Bridge, a regionally important Ohio River crossing which, if out of service, necessitates a detour of 55 miles. Of the 600,000 or so bridges in the United States, the vast majority are "boring" bridges with relatively short spans and lots of built-in redundancy. That means that even if a structural member is deteriorated, there is still enough load capacity too keep the bridge in service.
Bridge collapses are shocking in part because they're so rare. Regular inspections in keeping with the National Bridge Inspection Standards help keep them that way. Yes, we should be sure that bridge maintenance and inspection is adequately funded; yes, research into improved methods for structure evaluation and testing should continue - but there's no reason to panic over big numbers with scary-sounding sames that ultimately have little meaning from a safety perspective.
1. Federal Highway Administration: Bridge System Conditions - Explanation of Bridge Deficiencies
2. United States Code: 23 CFR 650.305 (2009).