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Chapter 21: Fundamentals of Signal Timing and Design: Pretimed Signals. Chapter objectives: By the end of this chapter the student will be able to:. Explain the basics of signal timing Know how to handle left-turn vehicles by various phase plans Define terms related to phasing
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Chapter objectives: By the end of this chapter the student will be able to:
1. Development of a phase plan and sequence
The process is not exact, nor is there often a single “right” design and timing for a traffic control signal.
The most critical aspect of signal design and timing is the development of an appropriate phase plan.
21.1.1 Treatment of left turns (the single most important feature that drives the development of a phase plan)
Fully protected phasing is also recommended when any ONE of the following criteria are met:
Compound phasing (protected-permitted) may be considered when LT protection is needed but none of these criteria are met. – Use compound phasing at less critical areas because it is a confusing phasing.
Phase diagram: Shows all movements being made in a given phase within a single block of the diagram.
Ring diagram:Shows which movements are controlled by which “ring” on a signal controller.
A “ring” of a controller generally controls one set of signal faces. Thus, while a phase involving two opposing through movements would be shown in one block of a phase diagram, each movement would be separately shown in a ring diagram.
Basic two-phase signalization
This works for either case of having a left-turn bay or not having a left-turn bay. If a left-turn bay is available, performance and safety increases.
Note how LTs are treated. One direction (WB in this case) has a short LT phase.
The term “phase” is loosely used sometimes. “Eight-phase” here is that it is possible to have 8 phases, but usually 4 phases as you see in the ring diagram.
NEMA no longer includes lead-lag option and this 8-phase scheme replaced it.
This order may have a problem.
This one, too.
LTs may be trapped in the intersection because there is no clearance time for LTs beyond the yellow interval. (sometimes there is AR – then possibility for rear-end collision)
Possible rear-end collisions for LTs because the LT driver might hesitate for an instant.
LTs are permitted, but dangerous.
(Source, pages 61-65, “Manual of Traffic Signal Design” by Kell & Fullerton, ITE)
An exclusive pedestrian phase is added. This was started in New York City by then Traffic commissioner Henry Barnes, hence called “Barnes Dance.” Not any more in NYC but you see right next to Clyde Building. Visit 900N & East Campus Dr.
If this street is a one-way street heading south, then it is not that bad (3 phases). But disallow “U-turn from the south approach into the diagonal link.
This was created by a data set collected in Salem, Oregon. It may not apply to other cities, but it is useful to make initial plans.
This figure shows a case of one-lane approach.
Note that this figure was eliminated in the 3rd and 4th edition of the text. But, I thought it is useful.
21.2.1 Change and clearance intervals
All red = clearance interval
Yellow = change interval
p.441 2nd edition: If there is no all-red interval, it is the driver’s responsibility to check if the intersection was cleared of traffic. “…over 60% is not aware of this legal responsibility. Also, 60% indicated that they did not bother to look for traffic from the conflicting street when given the GREEN indication.” -- Note that this statement was eliminated in the 3rd edition. So, this is not in the 4th edition, either.
To safely stop:
To safely clear:
Cannot stop or cannot clear (Xc>Xo)
When Xc = Xo, there is no dilemma zone - at least theoretically.
ITE took this part as the length of the yellow interval.
This part as the length of the all-red interval.
This part shows the effect of gravity on deceleration vector.
Note that where approach speeds are not measured and the speed limit is used, both the y and ar intervals will be determined using the same value of speed. (p.504 Left col.) Not desirable, though. See next page for Why?
a = deceleration rate (e.g., 10 ft/s2)
g = gravity (32.3 ft/s2)
G = grade of approach (in decimals)
t = perception-reaction time (1.0 sec)
S85 = 85th percentile speed or the speed limit, mph
Case 1: Practically no pedestrian (w = street width)
(S15 in mph)
Case 3: Some pedestrian traffic:
Case 2: Significant number of pedestrians OR where the crosswalk is protected by pedestrian signals
S15 = S – 5
S85 = S + 5
If LTs are more critical
S = average approach speed
D21.2.2 Determining lost times
l1 = 2 sec/phase
e = 2 sec/phase
i = Number of phases
(by HCM 2010)
Two factors require special attention:
Convert all turning demand volumes to equivalent through vehicle units (TVUs) first. Please note that adjustments for heavy vehicles are not done for initial signal timing because it gets too complex (see p.505, right column, 2nd paragraph).
The effect of turning vehicles are included in Vc by multiplying ELT and ERT as shown in Table 21.1 and 21.2 (Note these are different from the ones we use for computing fHV in capacity analysis.)
Interpolation not recommended
Can be interpolated.
Use Example 21-2
This simple method gives you the direction for detailed signal timing. This method using the formula for the “time budgeting” method as the basis.
Once the cycle length is determined, the available effective green time in the cycle must be divided (split) among the various signal phases in proportion to Vci/Vc.
Finding actual green interval values (Gi):
Gi = gi – Yi + tLI
Do the sample timing in page 507.
HCM 2000 (& 2010) requirements:
For WE > 10 ft
(width of crosswalk in ft)
For WE≤ 10 ft
Nped = No. of peds crossing per phase (cycle).
The sum total of 1st and 2nd term is WALKmin. The third term is FLASHING DON’T WALK.
If Gp > G + Y, (a) change the signal timing to satisfy this requirement, or (b) install pedestrian detectors (buttons) at the intersection. When the button is pressed, the controller will provide a G + Y equal to Gp during the next available green phase.
When Gp controls, make changes in green times to maintain the original ratio of vehicular green time. See pages 509 and 510.
Note that in this case Gp = G + y + ar