Civil Engineering Reference
In-Depth Information
Table 12.1 Maximum throughput volume (V
m
), critical speed (S
c
) and critical density (D
c
) for
freeways with different free-flow design speed (Sf)
f
)
Design speed
Maximum throughput volume
Critical speed
Critical density
S
f
(mph)
V
m
(pcphpl)
S
c
(mph)
D
c
ΒΌ
V
m
=
S
c
(pcplm)
75 and 70
2,390
53
45
65
2,340
52
45
60
2,290
51
45
55
2,250
50
45
Source
Approximate values estimated from Fig. 1
traf
c speed drops to from 75 or 70 to 53 mph, from 65 to 52 mph, from 60 to 51 mph,
and from 55 to 50 mph, respectively, depending on the initial free-
ow speed.
At these maximum throughput volumes (V/C = 1.0) and at their critical speeds
(S
c
), their critical density (D
c
) of 45 passenger cars per lane mile is reached. these
relationships are shown in Table
12.1
.
When critical density is reached, however, traf
fl
flow becomes unstable, vehicle
speeds are apt to drop unpredictably below their critical speed (S
c
), and productivity
also declines. This speed-
c
fl
ow pattern is shown in Fig.
12.2
for an urban freeway [
2
]:
Density contours (vehicles per lane per mile) along a freeway have been useful
in identifying the location and extent of high densities (i.e., congested
fl
ow). The
contours can also identify bottleneck points both upstream and downstream of a
given location. Density contours such as shown in Fig.
12.3
, have been developed
fl
Fig. 12.2 Speed
fl
flow data for an urban freeway.
Source
Reference [
2
], Fig. 5
