Civil Engineering Reference
In-Depth Information
Fig. 12.5 Estimated speed/density relationship for congested freeway flow.
Source
Scale same as
in Fig.
12.4
Fig. 12.6 Speed-density relationship for uncongested and congested flow for 70 mph freeway.
Source
Scale as in Figs.
12.4
and
12.5
Figure
12.7
shows how the throughput volume reaches its maximum of 2,390
passenger cars per hour per lane at a density of 45 cars per lane per mile.
Throughput volume then progressively declines to about 1,250 pcphpl as den-
sity increases to 100 vehicles per lane per mile, and to about zero
fl
ow at
165 pcplpmile.
Figure
12.8
shows the resulting speed-
fl
ow curve. With
increasing
volume, speed
drops minimally from its free-
ow speed of 70 mph until it reaches about
1,800 pcphpl [A]. As volume increases beyond this value, traf
fl
c speed becomes
unstable [B] and susceptible to sudden drops in speed and throughput volume [C].
