Civil Engineering Reference
In-Depth Information
Table 7.1 Sources of congestion
—
a national summary
Congestion causes
Recurring (%)
Non-recurring (%)
Bottlenecks
40
-
Poor signal timing
5
-
Traffic incidents
25
-
Work zones
-
10
Bad weather
15
-
Special events/other
5
-
Total
45
55
Source
Reference [
1
]
Table 7.2 Percentage contribution of recurring and nonrecurring causes to total delay, by area
type and size
Cause of delay
Large Urban
areas >1 m
a
Small urban areas
0.1
-
1.0 m
Rural
Recurring causes
Network
demand > capacity
29
-
37
20
-
26
0
Poor signal timing
4
5
7
13
2
-
-
Total recurring
33
42
32
33
2
-
-
Non-recurring
causes
Crashes
35
-
36
19
-
26
26
Breakdowns
6
-
7
6
-
10
25
Work zones
8
19
26
27
39
-
-
Weather
5
6
7
10
7
-
-
Special events/lack
of information, other
1
-
0
Total
non-recurring
58
67
67
98
-
a
Combine estimates for size classes 1
3 m and > 3 m.
Source
Reference [
2
]. Used by permission
-
It is important to note that these estimates refer to an area-wide average, and are
not intended for estimating conditions at speci
c highway corridors within the
urban area, as these must be estimated from actual experiences. For example, a
corridor with older highways and sub-standard designs (e.g., the I-278 corridor in
Brooklyn, NY), would experiences higher crash rates, so that the proportion of
congestion delay from traf
c incidents in the I-278 corridor would be higher than
that indicated in the above table.
This chapter describes the various bottleneck factors that contribute to recurring
and nonrecurring congestion.
