Adaptation Strategies for Managing Recurring Congestion—Adding New Capacity - Road Traffic Congestion - page 235

downstream roadways. Removal of a bottleneck, therefore, may transfer the delay
from upstream to downstream roadways.
For example, consider a bridge carrying traf

c to and from the central business
district (CBD) of a large city, where the morning commuter traf c demand destined

to the CBD and the evening traf c demand exiting the CBD exceed the capacity of

the bridge. In this case the bridge meters the morning traf c demand to the CBD

and increasing its capacity will increase the traf c congestion on the roadways

within the CBD. On the return trip, however, increasing the capacity of the bridge
enables traf

c demand exiting the CBD to be processed at a faster rate resulting in
lower congestion on CBD streets.
The bene ts of a bottleneck removal, therefore, need to be evaluated from the

standpoint of its impact on total trip delay and the possible transference of con-
gestion impacts from one area to another. Increasing outbound bridge capacity
would lower the intensity and duration of congestion on the CBD streets bene

ting
its businesses, residents and visitors; while increasing inbound capacity would
increase the intensity and duration of CBD congestion.
In this example if the goal is to improve traf c conditions in the CBD, the

strategy should be to remove the bridge bottleneck for outbound traf

c only.

17.2.2 Intersection Improvements

Intersections are a common source of traf c congestion when they process a large

number of con icting movements. As discussed in the previous chapter, often

congestion can be reduced by reducing the con

fl

ict points by providing turning
lanes within the existing roadway width, restricting turning movements, and/or
making streets one-way.
In many cases, however, adding new capacity is needed to reduce con

fl

fl icts and

reduce delay. This strategy entails the expansion and recon guration of intersec-

tions by (1) intersection widening to provide auxiliary turn lanes, (2) reducing
con

fl icts — e.g., using “ jughandles ” and “ indirect ” left turns, (3) replacing traf c

signals with roundabouts where traf c signals are inef cient in controlling con-

fl especially where more than two streets intersect, and (4)

separating the grades of con

flicting traf c fl ows —

fl icting fl ows.

17.2.2.1 Auxiliary Through Lanes [ 5 ]

Auxiliary lanes consist of roadway widening to increase the capacity of arterial
roads in suburban areas. They are provided upstream and downstream of the
intersection to serve left turn, through, or right turn volumes at heavily traveled
suburban intersections. Illustrative sketches of such treatments are shown in
Fig. 17.2 .