The Costs and Other Consequences of Traffic Congestion - Road Traffic Congestion - page 150

13.2.1 Components of Congestion Costs

The various components of travel cost in congested traf c are shown in Eq. 13.1 :

Travel Cost Per Vehicle in Congested Traffic

¼ ½ ð Additional Trip Time from Congestion Þ ð Value of Travel Time Þ

þ ½ Additional Fuel Cost From Congestion

þ ½ Cost of Additional Crashes

þ ½ Cost of Additional Air Pollution to Human Health

ð 13 1 Þ

:

The variables of this equation are discussed below in order:

1. Additional Trip Time
2. Value of Travel Time
3. Additional Fuel Consumption
4. Additional Crashes
5. Health Cost of Additional Air Pollutants.

13.2.1.1 Additional Trip Time

a. Based on Average Threshold Speed of Congestion

Additional Trip Time ¼ ð Trip Time at Congested Speed Þ

ð Trip Time at Threshold Speed Þ ð 13 : 2 Þ

where:

Trip Time at Congested Speed ¼ ð Congested Speed Þ ð Trip Length Þ Trip

ð 13 3 Þ

:

Time at Threshold Speed ¼ ð Threshold Speed Þ ð Trip Length Þ ð 13 : 4 Þ

Trip length is an important variable in the analysis of congestion costs. This
is because the cost of congestion is smaller for shorter trips and bigger for
longer trips.
b. Based on Time Variability
The travel time experience of most road users is not based on the average value
of congested travel times but instead it is based on the worst days when their trip
was unexpectedly delayed. Figure 13.1 shows that the 95 percentile travel time
rate is almost double the average travel time rate.