Civil Engineering Reference
In-Depth Information
7.3 Nonrecurring Bottlenecks
7.3.1 Introduction
Nonrecurring congestion results when the roadway
capacity
is reduced by
(1) incidents that remove one or more travel lanes from service, or cause drivers (on
both sides of the road) to slow down as they to observe the roadside activities
related to the incident; and by (2) headways that are increased by inclement
weather, work zones, or driver behavior. Another cause of nonrecurring congestion
is a surge in demand in excess of what the roadway can handle (e.g., the exit of
spectators at the end of a ball game).
In all cases there is an imbalance between roadway supply and travel demand. In
addition to the duration of the above events (e.g., road blockage or demand surge)
there is also delay during the recovery time until the normal traf
c operation
resumes.
7.3.1.1 Traffic Incidents
Traf
c incidents reduce roadway capacity and contribute to congestion. The amount
of delay depends upon the type/duration of the incident, the number of lanes
blocked by the incident, the response times to reach and clear the incident, and the
time needed for the roadway (freeway) to resume normal operation.
Traf
c incidents reduce roadway capacity [
1
]. Estimates of the amount of
freeway capacity available, as a function of number of lanes blocked by the inci-
dent, are provided in Table
7.4
, which shows that even when an incident is located
at the shoulder of the road it reduces its capacity.
Table 7.4 Freeway capacity available from incident conditions
Number of freeway lanes in
each direction
Shoulder
disablement
Shoulder
accident
Lanes blocked
One
Two
Three
2
0.95
0.81
0.35
0
N/A
3
0.99
0.83
0.49
0.17
0
4
0.99
0.85
0.58
0.25
0.13
5
0.99
0.87
0.65
0.40
0.20
6
0.99
0.89
0.71
0.50
0.25
7
0.99
0.91
0.75
0.57
0.36
8
0.99
0.93
0.78
0.63
0.41
Source
Reference [
1
], pp 1
9, Table 1
2
-
-
