Bottlenecks - Road Traffic Congestion - page 79

Fig. 7.20 Suggested speed
estimation curves as a
function of signal spacing and
V/C Ratio — Class II and III
arterials. Source Reference
[ 7 ], Fig. 7.11

Table 7.3 Percent increase in
travel time as a function of
traf c signals per mile

Signals per mile Percent increase in travel time

(two signals per mile as base) (%)

3.0 9

4.0 16

5.0 23

6.0 29

7.0 34

8.0 39

Source Reference [ 8 ]

c speed reduction is expressed
by a family of curves for various volume-to-capacity (V/C) ratios. For example, for
class I arterials, approximately 2 mph drop in speed results for every 0.1 increase in
the V/C ratio for roads with fewer than 4 signals per mile. But for roads with 8
signals per mile the drop in speed diminishes to a little over 1 mph for every 0.1
increase in the V/C ratio.
These curves show that traf

The effect of an increase in traf c volume on traf

c
volumes when the volume-to-capacity ratio is less than 0.8. Signal density has its
biggest effect of free-

c signal density has a greater effect on delay than traf

c demand
approaches or exceeds roadway capacity there is a drop in speed at all signal densities.
Using two traf

fl ow traf cat1 3 signals per mile. When traf

-

c signals per mile as a base, Table 7.3 provides estimates of the
percentage increase in travel time as the signal frequency per mile increases [ 8 ].
These relationships suggest that (1) the number of phases should be kept to a
minimum, and (2) spacing of signals should permit progression

fl flow to the maxi-

mum extent possible.