Week 7 Flashcards
What do traffic signals do
- Improve overall safety
❖ Angle and side collisions (which are more
severe) are often reduced
❖ However, rear-end collisions (which are less
severe) may increase - Pedestrian accidents usually are reduced
- The intersection’s capacity is increased
❖ Travel times are decreased
❖ Fuel consumption and environmental
benefits - Attempts to equalise the quality of service
for all or most of the approaching traffic
streams
❖ Assist in traffic management and other
objectives
4
Each possible trajectory of traffic flow is called a
Movement
Phases consist of two parts:
Running Part: The green signal duration, allowing permitted movements to proceed.
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Clearance Part: The amber and all-red signal durations. The amber signal warns drivers to prepare to stop, while the all-red signal ensures the intersection is clear before the next phase begins.
A signal phase is a
state of the traffic signals during which one or
more movements have the Right-of-Way at an intersection
Cycle Time
The time it takes to complete one full sequence of phases
Displayed Green Time (G)
The time period that green is displayed for a phase
All-Red Clearance Time
Time when all signals are showing red
Saturation Flow
The maximum flow that can be discharged from a
traffic lane when there is a continuous green indication
and a continuous queue on the approach
What is saturation flow affected by
Saturation flow can be affected by:
* Geometry
* Gradient
* Visibility
* Lane width
* Driver behavior
* Traffic composition
8
Minimum Green Time (Gm)
– This is an input to the design process
– Usually a default value for motor vehicle movements of 6 or 7
seconds is used (can be reduced to about 5 seconds for right-
turns)
– This is often a limiting factor for pedestrian movements,
determined by walking time across wide intersections
Definitions
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Intergreen Time, Ig
– The time between green times for consecutive phases
Intergreen = Amber Time + All-Red Clearance Time
Total Green + Total Intergreen = Cycle Time
Critical Lane
– It is defined as the lane that has the most intensive traffic.
– There will be one and only one critical lane in each signal
phase
Critical Lane Volume
– The traffic volume of this critical lane is called critical lane
volume.
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What is a “phasing diagram”?
A visual representation of the sequence of all phases within a signal cycle, illustrating the order in which different movements are given the right-of-way.
What are the three main types of traffic signals
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Pre-timed traffic signal: Operates on a fixed schedule with predetermined phase durations and cycle times, regardless of real-time traffic conditions.
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Semi-actuated traffic signal: Uses vehicle detectors on minor streets to adjust green times for the major street based on detected demand, while the major street generally has a continuous green signal unless actuation occurs on the minor street.
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Fully-actuated traffic signal: All approaches have vehicle detectors, allowing the system to dynamically adjust phase durations and cycle times based on real-time traffic flow detected on all approaches
Distinguish between “fixed-time” and “actuated” traffic signals.
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Back:
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Fixed-time: Operates on a preset schedule, ignoring real-time traffic variations (pre-timed signals fall in this category).
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Actuated: Uses sensors to detect traffic and adjusts signal timings dynamically based on real-time traffic conditions (semi-actuated and fully-actuated signals fall in this category)
List the four major steps in the signal design process.
1.
Phase design: Defining the sequence and combination of movements allowed to proceed during each phase, considering conflict minimization.
2.
Determine amber time and all-red clearance time: Calculating the durations for amber (warning) and all-red (clearance) signals to ensure safety and efficient traffic flow.
3.
Estimate the signal cycle time: Determining the total duration of one complete signal cycle, often using Webster’s formula.
4.
Allocate green time for each phase: Dividing the available cycle time among the different phases, considering traffic volumes and demand
How is amber time calculated?
mber Time = (SSD + W + L) / V, where:
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SSD is the stopping sight distance.
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W is the width of the intersection.
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L is the length of the vehicle.
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V is the speed of the vehicle (or the design spee
How is all-red clearance time calculated?
All-Red Clearance Time = W / V, where:
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W is the width of the intersection.
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V is the speed of the vehicle (or the design speed)
How is flashing red time calculated for pedestrian lights?
Flashing-Red Time = (W / V) - Gm, where:
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W is the width of the intersection (length of the footpath).
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V is the speed of a pedestrian (typically 1.2 m/s in Australia, adjustable based on demographics).
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Gm is the minimum green time for pedestrians.
Write Webster’s formula.
C = (1.5L + 5) / (1 - Σ(Vi/si)), where:
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C is the cycle time.
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L is the sum of intergreen periods.
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Vi is the critical lane volume for phase i.
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si is the saturation flow for phase i.
How is saturation flow estimated?
Saturation flow = 3600 / headway time, where:
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Saturation flow is in vehicles per hour per lane.
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Headway time is in seconds.
