Civil Engineering Reference
In-Depth Information
24.5.4 Traffic Productivity
Traf
c congestion can reduce the capacity of freeways and other roads.
(1) Traf
c volumes
increase, speed drops. The lowest speed where the throughput volume reaches
its maximum value is the
critical speed
c volume on a roadway determines the traf
c speed: as traf
so called because when speed
continued to drop below this value, the throughput volume of the roadway
begins to drop as well.
(2) The critical speed for freeways approximates 50 mph. Lower speeds result in
reduced throughput capacity.
(3) For arterial streets, critical speeds of about 10
—
15 mph have been reported.
-
c moves at below its critical speed there is a loss in
throughput volume which also increases in the
duration
of congestion.
Therefore, when traf
24.5.5 Crashes
Traf
c congestion increases the density of vehicles occupying the roadway (vehi-
cles per lane per mile of road increases). When vehicles follow each other at close
spacing they tend to change lanes more frequently
—
merging into crowded lanes to
exit or enter the roadway
—
increasing the risk and frequency of crashes.
24.5.6 Air Quality and Health
Traf
c congestion degrades air quality with direct consequences to human health.
Average emission rates are 2
-
3 times higher at speeds of less than 10 mph, than
they are at speeds between 20 and 80 mph. Relieving traf
c congestion is often
cited as an air quality and sustainability improvement strategy.
24.5.7 Congestion Costs
The cost of congestion can be measured for both personal travel and goods
movement, through the additional travel time, fuel consumption, and the additional
crashes incurred.
The personal travel hourly cost of congestion delay time is approximately
valued at 50 % of the hourly wage rate, while the hourly cost of business travel
approximates 100 % the hourly wage rate.
