Civil Engineering Reference
In-Depth Information
Table 2.5. Trends in Deficient Bridges
1992
1997
2002
2007
2011
Number of bridges
572,196
582,751
591,220
599,766
605,086
Structurally Deficient
118,698
98,475
81,437
72,524
67,526
20.7%
16.9%
13.8%
12.1%
11.2%
Functionally Obsolete
80,392
77,410
81,573
79,792
76,363
14.0%
13.3%
13.8%
13.3%
12.6%
Not deficient
373,106
406,866
428,210
447,450
461,197
65.3%
69.8%
72.4%
74.6%
76.2%
Source:
National Bridge Inventory
bridges facing increased demands to carry traffic? Though we do not have
reliable measurements of traffic exactly at bridges, we do know that through
2007 urban areas were indeed undergoing increased traffic congestion. That
observation comes from the
Urban Mobility Report,
a study prepared by the
Texas Transportation Institute and published in July 2009. Before accepting
the result, the attentive reader must ask what “congestion” means, since it
is by no means easy to define.
To gather their data, the Texas researchers studied conditions during
peak travel hours, which they defined as 6 to 10 a.m. and 3 to 7 p.m. These
are the hours during which about 50 percent of daily travel takes place—it
is the time when the most demand is placed on road infrastructure. They
then collected traffic data for these time periods at thousands of road seg-
ments in 439 urban areas.
For each lane in the road segments studied, they used computer pro-
grams to estimate travel times under free-flow conditions (no jams, break-
downs, crashes, or weather problems). With the collected traffic data, they
then divided actual travel times during peak hours by the theoretical travel
times under the free-flow conditions. The result was the “travel time index.”
If it were exactly “1,” it would mean that traffic moved at the free-flow rate.
But in all metro areas the index was higher than 1.
The Los Angeles metro area had the highest index—1.49—which
meant that travelers on the average spent 49 percent more time traveling
during peak hours than they would have under free-flow conditions. To exas-
perated Angelinos, the index may seem too low. But they must remember
that the index includes travelers who hit the road at 6 a.m. and managed
to escape the worst of the congestion.
