Bottlenecks - Road Traffic Congestion - page 82

Table 7.6 Effect of weather-
generated pavement
conditions on traf c speed

Condition Percent speed reduction (%)

Dry 0

Wet 0

Wet and snowing 13

Wet and slushy 22

Slushy in wheel Paths 30

Snowy and sticking 35

Snowing and packed 42

Source Reference [ 11 ]

Street Closures : These events result from emergencies or of planned events (mar-
athon, street fairs, visits by heads of state, etc.). Because they reduce roadway
capacity, travel speed drops.
Utility Cuts : In many cities utilities are located below the roadway surface, and
their repair often involves closing at least one lane to traf

c that reduces capacity

and travel speed.

7.3.1.5 Driver Behavior

Erratic and improper driver behavior can contribute to a reduction in traf c speed

resulting in congestion.
Examples follow:

a. Use of the passing lane by one slow driver reduces the speed of all drivers.
b. Drivers tend to slow down while passing an incident location in the opposite
direction (rubbernecking).
c. Loading or Unloading in moving lanes: the use of moving lanes by commercial
vehicles for loading and unloading reduces capacity and forces vehicles to slow
down as they change lanes. The same goes for bus drivers that don ' t pull into the
bus stop, and for taxi drivers who pick up or discharge passengers from the
moving lane.

7.4 Conclusion

As previously shown in Table 7.2 , the proportion of total delay attributed to
nonrecurring bottlenecks in urban areas far exceeds that from recurring bottlenecks.
As much as 2/3 of the total delay in large and small metropolitan areas is attrib-
utable to nonrecurring bottlenecks.
Since nonrecurring congestion is often experienced at the same time and loca-
tions where recurring congestion occurs, the severity of nonrecurring delay is