Civil Engineering Reference
In-Depth Information
The key causes of intersection congestion include:
an insuf
cient number of travel lanes on intersection approaches
the lack of exclusive lanes of adequate length for right and left turns
heavy traf
c volumes and turning movements on the various con
fl
icting
approaches
heavy pedestrian and bicycle movements that con
fl
ict with and impede and
impeded traf
c
fl
ow.
7.2.2.3 Traffic Signals
Because traf
c signals control con
fl
icting movements they account for much of the
traf
c delay along streets and roads. Their location, phasing, and timing can sub-
stantially
increase
congestion when:
the total green time per signal cycle must be shared by con
fl
icting traf
c streams
right-turns con
fl
ict with heavy pedestrian volumes
left-turns operating from a lane shared with through traf
c can block through
vehicles. When there is one left-turn per cycle, about 40 % of the through
vehicles in the shared lane are blocked. When there are three left-turn vehicles
per cycle, about 70 % of through traffic is blocked. When protected left turn
lanes are provided, there is generally no impedance to through traf
c moving in
the same direction
Left turns with exclusive turn lanes must share the green time with the through
traf
c in the opposing direction.
Traf
c signal location, spacing, and timing de
ciencies commonly include:
a. Placing signals where they do not
fit the progression pattern reduces the width of
the through (or green)
—
band (Fig.
7.18
).
b. Although ef
cient progression can be maintained by increasing the green time on
the major street, but this condition would require a reducing the green time on the
cross street with a corresponding increase in delay to cross street traf
c (4).
c. Providing an excessive number of phases such as a
pre-timed
exclusive
pedestrian phase where there are few pedestrians crossing a highway.
d. Using cycle lengths that are too short to serve peak traf
c demands can result in
excessive delay.
e. Using cycle lengths that are too long (e.g., over 2 min) make signal coordination
dif
cult to achieve.
f. Operating closely spaced signals that are not coordinated.
g. Operating obsolete traf
c signal control systems that limit the ability to establish
c responsive signal coordination.
h. Locating signals at irregular intervals that limit or preclude coordination.
i. Placing signals too close together, thereby limiting effective coordination and
resulting in frequent stops.
time-of-day or traf
