Civil Engineering Reference
In-Depth Information
Fig. 8.6
Speed versus flow rate for freeways.
Source
Reference [
17
], 2010 HCM
becomes unstable and the speed can drop suddenly to reach stop-and-go conditions.
The values of Fig.
8.6
represent
“
ideal
”
conditions in terms of roadway and inter-
change design
conditions that seldom exist in many urban areas.
The resulting level of service standards (A
—
-
F) for freeways (shown in Fig.
8.1
),
corresponding to traf
c density, are summarized in Table
8.7
.
For signalized and unsignalized intersections, the standards are established by
the delay time experienced by an average vehicle before crossing the intersection
(Tables
8.8
and
8.9
).
For intersections that are not signalized, the delay time standards are given in
Table
8.9
.
Level of service metrics corresponding to speed thresholds for automobiles on
city streets are given in Table
8.10
. These criteria relate level of service to the
observed travel speeds as a percentage of the base free-
ow speeds. The thresholds
are generally consistent with of the previously established congestion criteria.
fl
Table 8.7
Level of service
criteria for freeways
Traf
c density (passenger cars/lane/mile)
Level of service
11 or less
A
>11
-
18
B
>18
26
C
-
>26
35
D
-
>35
-
45
E
>45
F
