Civil Engineering Reference
In-Depth Information
Chapter 7
Bottlenecks
7.1 Introduction
Travelers and freight movers experience congestion as a result of capacity
de
ciencies in the roadway system. These congestion problems occur whenever the
arrival rate (demand) exceeds the vehicle departure rate (capacity). The demand-
capacity imbalance is manifested by a number of conditions: (a)
recurring bottle-
necks
are caused by topographic and physical barriers to movement; the disconti-
nuity of the road network; design and operational de
ciencies; (b)
nonrecurring
bottlenecks
are created by incidents, surging demand, inclement weather, work
zones/street closures, and driver behavior.
When recurring and nonrecurring bottlenecks happen at the same time their
impact on delay is at its worst. The amount of delay caused by recurring events and
by nonrecurring events has been estimated by several sources but the estimates
provided are not clear as to the amount of congestion produced when recurring and
nonrecurring delays overlap.
Table
7.1
shows a national estimate of the causes of recurring and nonrecurring
congestion delay for US freeways and expressways.
These estimates represent rough approximations from past and ongoing studies.
They show that
nonrecurring
congestion generates a larger share of delay than
recurring congestion (55 vs. 45 %). Incidents cause 45 % of nonrecurring delay and
recurring bottlenecks are the most common cause of recurring delay (89 %).
Lockwood [
2
] breaks down recurring and nonrecurring delay by area type and
size of urbanized area. Table
7.2
shows the percentage contribution of recurring and
nonrecurring causes to total delay by size and type of area.
Thus the delay in rural areas is predominantly of the nonrecurring type (97 %),
while in urban areas nonrecurring delay is a much smaller share of total delay. This
share decreases slightly as the size of the area increases (58
72 % in small urban
-
areas versus 55
69 % in areas greater than one million people).
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