Civil Engineering Reference
In-Depth Information
3.3 Traffic Congestion in the 21st Century
Today congestion is found on circumferential and cross-town roadways as well as
radial highways. The growth and dispersion of residences and work places re
ects
the continued shift of people and jobs to suburban settings; the rise of of
fl
ce
buildings and of edge cities [
8
] along freeways/beltways; and the decline of pop-
ulation and employment in older central cities.
Longer trip lengths and less transit use place greater traffic pressure on many
arterial roads and freeways. Consequently the percentage of urban freeways that are
congested has grown substantially since 1970. Studies by the Federal Highway
Administration show that in 1970 about 30 % of the urban freeways had volume-to-
capacity ratio exceeding 0.77; by 1990 more than 50 % of the nation
s freeways
exceeded this ratio [
9
]. In Houston, only 10 % of the freeways carried more than
15,000 vehicles per lane in 1980, but just 6 years later (1986) 60 % of the freeways
carried more than 15,000 vehicles per lane [
10
].
Congestion is most acute in urban areas of more than two million people. Today,
freeway congestion is found along beltways and many of the radial freeways
leading to them. Congestion on freeways can often extend 15
'
25 miles from the
city center during peak periods, and at times (as in Los Angeles) extends throughout
the day.
-
3.3.1 Congestion in Travel Corridors and at Bottlenecks
Area-wide metrics of traf
c congestion, while useful to describe trends, do not
re
ect the severity of congestion along major travel corridors and bottlenecks where
traf
fl
c moves very slowly for long periods of the day.
The magnitude of metropolitan area traf
c congestion is perhaps best measured
along freeway travel
corridors and at bottlenecks
. These measurements are con-
ducted by INRIX annually [
11
].
3.3.1.1 Corridors
INRIX ( [
11
] p 7, 22) de
nes a congested corridor as follows:
Recurring congestion
1
has to occur on multiple road segments totaling at least
3 miles in length.
At least one segment must be congested 10 h per week, on average, and
All road segments in the corridor must have at least 4 h a week of congestion, on
average.
1
Where traf
c speed is lower than the free-flow speed.
