Civil Engineering Reference
In-Depth Information
17.2.1 Bottleneck Reduction/Removal
A
“
bottleneck
”
results along heavy traveled roads where there is a severe imbalance
between traf
c demand and roadway capacity. Its impact on delay can vary widely
in intensity, extent, and duration.
17.2.1.1 Causes
”
and by the convergence of major roadways on approaches to bridges and tunnels.
They also result from inadequate freeway geometry such as closely spaced freeway
ramps, left-hand freeway entry and exits, short weaving distances, and inadequate
interchange design.
Along arterial streets, bottlenecks are usually created by complex intersections
with high traf
“
Bottlenecks along freeways and arterial streets are usually caused by
lane drops
c demand resulting in short
“
green
”
times, and long
“
red
”
times for
each traf
c movement.
17.2.1.2 Consequences
Recurring bottlenecks account for 40 % of congestion delays. Creating
additional
capacity is an essential component of a comprehensive congestion relief program to
relieve the system-wide congestion impacts of bottlenecks.
A study prepared for the American Highway Users Alliance estimated that
“
improvements to the 166 most serious bottlenecks nationwide including traf
c
operations, demand management, and capacity expansion could signi
cantly reduce
delays, crashes, and air pollution and result in signi
cant cost savings
”
[
4
].
17.2.1.3 Bottleneck Relief Strategies
The corrective strategies will vary individual circumstances, including location, type,
and severity of congestion. They include freeway widening and reconstruction of the
main travel lanes and interchanges, and ramp widening, metering, or ramp closure.
Improvements along arterial streets include intersection widening and simplifi-
-
cation of intersecting
flows, grade separation, and new controlled-access bypass
routes around congested business centers.
fl
17.2.1.4 Implications
The congestion bene
t of bottleneck removal, however, must be evaluated for the
entire roadway system
—
not just for the bottleneck location. Bottlenecks create
delays for traf
c upstream of the bottleneck, but they also meter traf
c demand at
