The Impacts of Congestion on Roadway Traffic Productivity - Road Traffic Congestion - page 137

traf c demand volume along the roadway at a given time; and (5) the capacity of the

road to serve the demand.
Traf

c volume increases, speed drops.
The speed at which volume reaches its maximum value is the critical speed

c speed varies with traf c volume: as traf

so
called because when speed continues to drop below this critical value the
throughput volume of the roadway begins to drop as well. Therefore the critical
speed becomes a useful metric in achieving the goal of maximizing/protecting
roadway productivity, and it is a useful traf

—

c congestion management tool.

12.3 Freeway and Expressway Productivity

12.3.1 Introduction

Figure 12.1 shows the relationship between average speed and average rate of
throughput volume for ow speeds: 75, 70, 65, 60,

and 55 mph as set forth in the 2010 Highway Capacity Manual, Chap. 11 (2).
These speed-

five freeways with different free- fl

fl ow curves indicate that traf c speeds on freeways do not begin to

drop perceptibly from free flow speed until their volume reaches about 80 % of their

maximum throughput volumes or when their respective volume-to-capacity ratio
(V/C) = 0.8. When freeways reach their maximum throughput volumes their free

fl

fl ow

Fig. 12.1 Typical speed- ow curves for freeways when traffic density is below 45 passenger cars

per lane-mile. Source Reference [ 1 ], highway capacity manual 2010, Chap. 11

fl