Lec 15, Ch.8, pp.291304: Signal Timing Objective

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Lec 15, Ch.8, pp.291-304 Signal Timing
(Objective)

• Be able to state the objectives of signal timing
• Learn by heart signal-timing related terms (very
  important)
• Understand how the yellow interval is determined
• Know how to design signal timing for a fixed
  (pretimed) signal at an isolated intersection
  using the Websters model

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What we cover in class today

• Terms relating to signal timing
• Objectives of signal timing
• Concept behind the determination of yellow
  interval dilemma zone
• Determining cycle length for a pretimed,
  isolated signal using the Webster method
• Allocating green time to each signal phase

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Terms relating to signal timing
Cycle length Phase Interval Change
interval All-red interval (clearance interval)
Controller
Phase B
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Terms relating to signal timing (cont)
Offset
All signal analyses are based on through
passenger cars. This way the analyses became
portable to any sites. Then you need conversion
of other-than-passenger cars and
other-than-through vehicles. Hence we need
Split phase
Phase 1A
Phase 1B
Passenger car equivalent (PCE)
Turning movement factors
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Terms relating to signal timing (cont)
Peak-hour factor (PHF) A measure of the
variability of demand during the peak hour. The
smallest time period used for traffic analyses is
15 min.
If PHF is known, Design hourly volume (DHV) is
computed by DHV (peak-hour volume)/PHF
412
390
380
375
Volume during peak hour
PHF
4 x Vol. During peak 15 min within peak hour
375 380 412 390
0.945


4 x 412
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Terms relating to signal timing (cont)
Lane Group consisting of one or more lanes on an
intersection approach and having the same green
phase Critical Lane Group the lane group that
requires the longest green time in a phase. The
critical lane group determines the green time
that is allocated to that phase. Saturation flow
rate the flow rate in veh/hr that the lane group
can carry if it has the green indication
continuously (see eq. 8.3 for the formula used by
the Highway Capacity Manual 2000.)
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Terms relating to signal timing (cont)
Once you have adjusted for truck and turn
movement factors, you are ready to design
phasing. One hint if the adjusted left turn
passenger car equivalent volumes is greater than
120 vph, we usually need a left-turn phase, which
means that we need a left-turn bay. Up to that
value, the yellow interval can provide enough
time for LTs. There are a few methods to deal
with left-turns. In this class you need to know
only the methods shown below (i.e. protected
left-turn phases) and in Example 8-5. All others
will be discussed in CE562. It is a complex issue.
130
600/2300
140
576/2288
?A
?B
?C
?D
Critical movement The maximum volume (vph) in a
phase.
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Objectives of signal timing

• Reduce the average delay of all vehicles
• Reduce the probability of accidents

Minimize the possible conflict points by
assigning the right of way to different traffic
movements

• Two conflicting objectives
• More phases, less conflict
• More phases, more lost time

• So, if at all possible, use
• 2 phases
• Short cycle length

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Yellow interval Dilemma zone
X0 the min. distance from the intersection for
which a vehicle traveling at the speed limit u0
during the yellow interval cannot go through the
intersection without accelerating
Distance traveled during yellow interval
Where tmin yellow interval
Xc the distance within which a vehicle traveling
at the speed limit (u0) during the yellow
interval cannot stop before encroaching on the
intersection.
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Yellow interval Dilemma zone (cont)
For the dilemma zones to be eliminated, X0 Xc
Comfortable deceleration rate a 0.27g
Solve for the yellow interval
If the effect of grade is added
G grade in decimals
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Cycle lengths of pretimed signals by the Webster
Method
Optimal cycle length C0 by the Webster method
L Total lost time per cycle (sec), usually you
lose 3 seconds per distinct phases. Yi Max
value of the ratios of approach flows (called
critical movement) to saturation flows for all
lane groups using phase i, qij/Sj ? Number of
phases Vij Flow on lane groups having the right
of way during phase i Sj Saturation flow on
lane group j for a through lane, about 1900
pcphgpl
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Cycle lengths of pretimed signals by the Webster
Method (cont)
Lost time for phase i
Total lost time
R Total all-red during the cycle
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Green time allocation
Available total effective green time, Gte C - L
Distribute effective green to each phase i by
And the actual green time for each phase i is
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Minimum green time
At an intersection where a significant number of
pedestrians cross, it is necessary to provide a
minimum green time that will allow the
pedestrians to safely cross the intersection.
After allocating green time, you must check with
the minimum green time for each phase.
For WE gt 10 ft
For WE 10 ft
At an intersection where only a few pedestrians
cross, this will be the length of green time for
pedestrian actuated green.