Civil Engineering Reference
In-Depth Information
8.3.4 Congestion Thresholds Established by Transportation
Agencies
—
Three Examples
8.3.4.1 New York Metropolitan Transportation Council (NYMTC)
The New York Metropolitan Transportation Council [
8
], identi
es congested links
in the highway network primarily using the Demand Volume-to-Capacity criterion
(D/C ratio), calculated by their traf
c assignment model.
Based on this criterion, and using the average of a 4-hour weekday morning peak
period, NYMTC established three congestion thresholds as shown in Table
8.11
.
For demand to capacity (D/C) ratio of 0.8 or less, there is co congestion but
where the D/C ratio exceeds 1.0, severe congestion occurs.
8.3.4.2 Washington State Department of Transportation [
18
]
Congestion thresholds are established as 75 % of posted speed limits. For example:
For urban Freeways with a speed limit of 60 mph, the congestion threshold
speed = 45 mph.
For arterial streets with a posted speed limit of 40 mph, the congestion threshold
speed = 30 mph.
8.3.4.3 Quebec Ministry of Transportation [
19
]
Congestion thresholds are established as 60 % of the posted speed limit.
8.3.5 Applications
The preceding examples show that urban travelers expect and accept a certain
amount of congestion during periods of the day. Congestion becomes unacceptable
when it exceeds a threshold value.
While the above examples re
ect the need to establish realistic congestion
thresholds, local threshold values are not universal: they vary by type of area, time
of day, and type of facility.
fl
Table 8.11
Demand-to-
capacity ratio thresholds for
congestion determination [
8
]
Demand-to-capacity ratio
Congestion determination
D/C = 0.8
No congestion
D/C = 1.0
Congested
D/C > 1.0
Severely congested
