Civil Engineering Reference
In-Depth Information
Table 12.3 Free-flow speed
and critical speed at
maximum throughput volume
for arterials of different free-
flow speeds
Average free-
fl
ow speed (Sf) of
Critical speed (Sc) at Vm
(V/C = 1.0)
arterial streets
40
17 mph
30
14
20
11
15
9
12
8
Source
Figure
12.9
12.4.2 Implications
The preceding speed-
fl
ow relationships lead to the following implications:
1. Although Fig.
12.9
does not show what happens to throughput volume when
traf
c speed drops below its critical value (as shown for freeways), it may be
assumed that for arterial streets throughput volumes decrease when speeds drop
below their critical value.
2. Therefore critical speed value can be used as the productivity-based threshold
speed because arterial streets will reduce their throughput productivity when
they operate at speeds lower than critical speed.
3. In these cases sustained traf
c demand will exceed the throughput capacity
resulting in growing queues (spillback) which will further reduce speeds along
the roadway
—
causing additional productivity losses.
12.5 Conclusions
Traf
c demand that exceeds the designated capacity throughput of freeways and
arterial streets causes congestion that leads to unstable traf
c operations and lowers
the roadway
c throughput below its designated value. In these cases it is
extremely important to reduce losses in capacity throughput due to congestion.
Available strategies that can accomplish this goal are described in Part 3 of the
topic.
'
s traf
References
1. Transportation Research Board (2011) Highway capacity manual 2010. s.n, Washington DC
2. Texas Transportation Institute. Quality control procedures for archived operations traf
c data:
synthesis of practice and recommendation.
http://www.fhwa.dot.gov/policy/ohpi/travel/qc/rec_
