The Costs and Other Consequences of Traffic Congestion - Road Traffic Congestion - page 149

In addition, congested speeds will increase travel time variability with its neg-
ative impacts on the level of service experienced by passengers. The resulting
consequences include uneven vehicle spacing that produces uneven vehicle loads

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with the lead bus in the platoon overcrowded and the following vehicle nearly
empty.

13.2 Calculating the Costs of Traf c Congestion

Traf c congestion costs includes the value of the extra time spent in congested

traf c, the extra fuel consumed, as well as the additional crashes that result, and the

additional amount of air pollutants that are generated.
The assumptions for baseline travel conditions for congestion cost calculations
often assign a cost value to the difference between free-

fl ow travel speeds and actual

travel speed. This difference is often referred to as “ lost ” time or travel “ delay. ”

approaches, when applied in large urban areas, can be
misleading because they do not recognize that, in large urban areas, congestion is
the outcome of higher land use density

Such “ cost of congestion ”

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itself the successful result of other urban

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policies and a somewhat lower threshold speed is a more realistic approach to

quantifying congestion (as was discussed in Chap. 8 ) .
Since travel demand is derived from social and economic activities (which add
value to society), the additional travel time from increased demand is the natural
consequence of satisfying increasing demand. Empty cities are not generally

considered successful cities; nor should empty roads

“

[ 11 ].
What should be the baseline for calculating congestion costs? The European
Conference of Ministers of Transport [ 11 ] notes that

”

“ the impacts of congestion are

not abstract — they must be linked to roadway users ' experiences and expectations.

Instead of attempting to calculate the of congestion from a theoretical

viewpoint, it may be more realistic and more productive to compare current levels
to and costs of congestion with past (and expected future) levels.

“ overall cost ”

This approach to
calculating congestion allows assessing the extent to which congestion is reducing
travel time reliability and accessibility to urban facilities and services.
An alternative approach in calculating congestion costs is by establishing
baseline congestion threshold speeds similar to the values suggested in Chap. 8 ,
where the concept of user acceptance of threshold congestion speeds were inden-
ti

”

ed for different size of urban areas, road types, and time of day; and in Chap. 12 ,
where the concept of “ critical speed ” was discussed.