The Impacts of Congestion on Roadway Traffic Productivity - Road Traffic Congestion - page 144

are best suited for evaluating speci c locations. Many of the equations are complex

and require computerized analysis.
This section, focuses on a general assessment of how traf

c speed on arterial

roads is impacted by the volumes they carry.
Figure 12.9 shows five types of arterials whose free- fl ow speed ranges from 40

to 12 mph. Streets with low free- fl low speeds (between 20 and 12 mph) are included

in the figure, although they are unlikely to function as “ arterial streets. ”

c volume increases, speeds generally decreases until a
critical speed is reached and throughput volume reaches its maximum value
(V/C = 1.0). The heavy lines are superimposed on the initial analysis to give a
working approximation of how speeds decrease as traf

It can be seen that as traf

c volumes increase. When
the volume-to-capacity ratios are less than 0.60, the speeds change very little. As
volume-to-capacity ratio increases, there is a sharp decline in speeds.
Table 12.3 summarizes the relationship between free-

fl ow speed (Sf), and the

critical speed (Sc) for maximum throughput volume (Vm) — reached when

V/C = 1.0, for each class of arterial streets in Fig. 12.9 .

Fig. 12.9 Speed and volume/capacity ratios for arterial streets. Source Reference [ 4 ], p 30, Fig. 3
and 6