Civil Engineering Reference
In-Depth Information
Table 12.2 Percent loss in throughput volume for freeway speeds below critical speed of 53 mph
Average
speed (mph)
Throughput
volume (pcphpl)
% Loss in throughput
volume (from maximum)
Incremental loss in
throughput volume (%)
Critical
Speed = 53
2,400
0
-
50
2,320
3
3
40
2,150
10
7
30
1,850
23
13
20
1,500
38
15
10
1,050
56
18
0
0
100
44
Source
Calculated
into an adjacent lane for a safe lane change needed to maintain a desired speed. As a
consequence some vehicles would be delayed by their inability to pass slower-
moving vehicles: drivers who cannot make the lane change are forced to drop its
speed if the car in front has reduced its speed. This kind of driver response is
multiplied in a chain reaction of speed reductions as vehicles come closer to one
another. When this occurs the traf
c throughput volume of the roadway gets pro-
gressively smaller, with a loss rate of 20 vph [2,390/(165
45)] for each unit
increase in density above 45 vplm (Fig.
12.8
); and when density reaches its max-
imum (Dj) value, a stop-and-go traf
-
c movement prevails.
The magnitude of the loss in throughput volume for various values of speed
below its critical value, is shown in Table
12.2
. It may be seen that the incremental
loss in throughput increases at a faster rate with each incremental loss in speed.
12.4 Arterial Street Productivity
Arterial streets, in contrast to the uninterrupted
fl
flow along freeways, involve
“
stop
and go
operations. Throughput volumes and speeds, are limited by various
interruptions along these facilities: signalized intersections result in stop and go
traf
”
c that result in delay. Midblock interference from parking movements and
other side frictions further impact speeds.
12.4.1 Analysis
The key determinants of travel speed include the frequency, coordination, and
timing of traf
icting movements at major intersec-
tions, and the number of lanes available on each intersection approach.
The 2010 Highway Capacity Manual (Volume 3) contains detailed procedures
for analyzing intersection and roadway performance [
1
]. The analysis procedures
c signals, the volume and con
fl
