COUNTING OUR BRIDGES - Bridges - page 13

whether the lanes are wide enough; bridges having lanes that are too narrow
by modern standards are considered obsolete. If clearances underneath for
road traffic are too low by modern standards; if emergency road shoulders
are insufficient or nonexistent; or if the approach roads to the bridge are
subject to flooding or have curvature that is too sharp—for any of these
reasons, too, a bridge is considered functionally obsolete.
So how do American bridges stack up? In making a judgment, we
have to keep in mind that the data is collected by state agencies, which
are required to use the same data when asking for federal highway funds.
Following NBI instructions, a state official would have to list a bridge as
structurally deficient even if the defect does not pose a danger of collapse,
or list a bridge with narrow lanes as obsolete even if daily users consider it
to be just fine. Then again, some of the deficiencies can be serious indeed.
The result is that 11 percent are structurally deficient and 13 percent
are obsolete. Altogether 24 percent of the nation's bridges have one short-
coming or the other or both (table 2.4). It's hard to know whether to read
this result as good news or bad news.
The good news is that the percentage of deficient bridges has been
declining (table 2.5). Structural deficiency has been dropping steadily from
20.7 percent of bridges in 1992 to 11.2 percent in 2011. Reasons may include
increasing quality of the bridge stock brought about by new construction, and
better maintenance and inspection. Over the same period, functional obsoles-
cence has remained fairly steady, fluctuating at about 13 percent of bridges.
Despite improvements, 24 percent of bridges were still flawed in one
way or another in 2011—that's almost 144,000 bridges! Now the bad news:
the bridges built in the 1960s and 1970s are reaching an advanced age, sug-
gesting an accelerating rate at which bridges will become deficient in the
coming years (unless ever more is spent on keeping them in good repair).

IS TRAFFIC CONGESTION INCREASING?

A bridge may have to be upgraded or replaced, or an additional bridge
may have to be built, for a reason other than deficiency: because it can-
not serve the growing traffic pressure (i.e., it is functionally obsolete). Are

Table 2.4. Deficiency in Bridges, 2011

Not Deficient Structurally Deficient Functionally Obsolete Total

461,197 67,526 76,363 605,086

76% 11% 13% 100%

Source: National Bridge Inventory