Bottlenecks - Road Traffic Congestion - page 66

7.2.1.2 Design Deficiencies

Traf c bottlenecks are also the outcome of the geometric street layout as well as

design de ciencies of critical sections/locations of the street network.

Street Network Geometry

City street patterns are an outgrowth of each city ' s history, geography and public

policy.

1. Pre-automobile Cities Streets in the central parts of many cities predate the

automobile and therefore are not designed to accommodate motor vehicle traf

—

c.

Typically they have short, irregular blocks with insuf cient capacity in the peak

hours for storing vehicles waiting for a green signal. This feature makes these
streets prone to spill back traf

cant congestion.
2. Many of the post WWII suburban developments have discontinuous street
networks and continuous streets that are spaced too far apart with the effect of
increasing the number of lanes in each street. This condition concentrate high
traf

c creating signi

c demand volume where these major streets intersect creating congestion
delays.
3. Washington , DC , streets are largely part of the L ' Enfant Plan for the National
capital. Its combination of multi-direction radial streets superimposed on a
rectangle grid creates many complex intersections commonly resulting in traf

c
congestion. Similar street plans were later adopted in Buffalo, Detroit, and
Indianapolis.
4. Converging radial streets are common in many older cities. This pattern often
results in peak hour congestion from converging traf c that exceeds the capacity

of the intersection.
5. Several cities have a diagonal street system superimposed on a grid. Chicago s

for example, has historic plank roads that create six-leg intersections where they
cross the grid streets. Historic Broadway in Manhattan cuts across the north-
south and east-west grid streets creating complex intersections where it crosses
the grid

'

— resulting in reduced intersection capacity.

Facility Design De ciencies/Constraints

Bottlenecks are created whenever any of the following conditions exist in the road
network:

￿ Lane Imbalance: At merge areas, at bridge and tunnel crossings, and where

several roadways converge without corresponding increase in travel lanes can
create extensive backups and congestion during busy travel periods.