Civil Engineering Reference
In-Depth Information
15.2 Incidents
Traf
c incidents reduce roadway capacity. Estimates of the amount of freeway
capacity reduction, as a function of number of lanes blocked by the incident, are
provided in Table
15.1
. This table shows that an incident reduces freeway capacity
even when it is located at the shoulder of the roadway, and it is not physically
blocking a lane. A roadway incident reduces the capacity of the roadway to various
degrees depending on the number of lanes on the roadway, the number of lanes
blocked by the incident, and the duration of the incident.
The factors involved in determining the amount and duration of delay resulting
from an incident are illustrated in Fig.
15.1
. They are described to show the
intensity (amount of delay), duration (hours of congested conditions), and
extent
(number of vehicles delayed) of congestion caused by the incident. It can be seen
that the duration of delay to the impacted traf
c can substantially exceed the
duration of the incident.
De
nition of terms in Fig.
15.1
:
t
0
= time when incident occurs;
t
1
= time when incident is detected;
t
2
= time when incident is reported to the traf
c management center (TMC);
t
3
= arrival time of
first responders with the means to restore roadway capacity;
t
4
= time when capacity is restored (all lanes are reopened);
t
5
= time when traf
flow is restored to conditions prevailing before the incident
with no demand reduction;
t
6
= time when traf
c
fl
c
fl
flow is restored to conditions prevailing before the incident
but traf
c demand is reduced by ramp closings or by diverting traf
c to other
roads via dynamic traf
c information devices;
(t
1
-
t
0
) = time elapsed after incident occurs and when it is detected;
(t
2
-
t
1
) = time elapsed after the incident is detected and when it is reported to the
TMC;
Table 15.1
Percentage of freeway section capacity available as a function of incident condition
and number of freeway lanes
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
