41.8 - Problems
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Interactive Audio Lesson
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Effective Green Time
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Let's delve into the concept of effective green time. Effective green time, denoted as g, is the total time the signal is actually green for vehicles to move, accounting for start-up and clearance lost times. Can anyone recall how we define effective green time?
Isn't it G + Y - L, where G is the green time, Y is the yellow time, and L is the lost time?
Exactly right! Effective green time helps us measure how much time is available for cars to pass through an intersection. Now, if we have G as 30 seconds and Y as 5 seconds, with lost time being 3 seconds, what would that make effective green time?
That would be 30 + 5 - 3, which equals 32 seconds.
Correct! Always remember, g is crucial for calculating lane capacities. Great job!
Saturation Flow Rate
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Now, let's discuss saturation flow rate. The saturation flow rate, denoted as s, is the maximum rate at which vehicles can pass through an intersection per lane during green time. What factors do you think affect saturation flow rate?
I believe it depends on the headway and possibly the type of vehicles passing through?
Spot on! The saturation headway, which is the time interval between successive vehicles passing a point, is key here. If we assume a saturation headway of 2.0 seconds, how would we calculate the saturation flow rate in vehicles per hour?
We would use the formula: s = 3600/h, so that would be 3600/2, which equals 1800 vehicles per hour.
Excellent work! This rate helps us determine how many vehicles can safely go through at peak times.
Cycle Length Calculation
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Let's move on to cycle length. The cycle length is critical in signal timing and traffic flow management. How do you think we derive this calculation?
Is it based on the total lost time and effective green time?
Correct! The cycle length can be derived using the formula: C = (N * t)/(1 - V/C) for N phases. If we are given start-up lost time of 2 seconds for 2 phases, how would you express this?
That means we have a total lost time of 4 seconds, which influences the cycle length for each phase.
Exactly! Cycle length is crucial for ensuring that we don't experience traffic bottlenecks.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
In this section, various problems are presented that test the understanding of traffic signal design principles, such as calculating capacity, cycle length, and effective green time, allowing for practical application of the theory discussed in the chapter.
Detailed
Problems in Traffic Signal Design
This section elaborates on the challenges faced in traffic signal design through specific problems that require application of theoretical principles. Problems discussed cover scenarios such as calculating effective green time, start-up and clearance lost times, saturation flow rates, and ultimately, the capacity of signalized movements. The formulation of these problems is aligned with prior content discussed in the chapter, ensuring a comprehensive focus on the practical application of traffic signal design concepts, including key calculations for determining capacity and cycle length in traffic scenarios.
Key Concepts
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Effective Green Time: The time a signal is green minus start-up and clearance lost times.
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Saturation Flow Rate: Maximum vehicle passage rate through a signalized intersection.
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Cycle Length: Duration for the entire series of traffic signal phases.
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Lost Time: Non-utilized time at an intersection.
Examples & Applications
If a signal has 30 seconds green time, 5 seconds yellow, and 3 seconds lost time, effective green time is 30 + 5 - 3 = 32 seconds.
With a saturation headway of 2 seconds, saturation flow rate can be calculated as s = 3600/2 = 1800 vehicles/hour.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Green, yellow, red we see, time to move, wait patiently; lost time means pause, before we go, effective green helps our traffic flow.
Stories
Imagine a busy city intersection where the lights help the cars decide when to move. Each light change is a dance, with cars swirling around but needing careful timing to avoid crashes. Ensure effective time is always calculated!
Memory Tools
To remember the flow: G (green) + Y (yellow) - L (lost) = g (effective green).
Acronyms
GYC - G - Green Time, Y - Yellow Time, C - Clearance Time.
Flash Cards
Glossary
- Effective Green Time
The net amount of time that a traffic signal is green for vehicles, factoring in lost time.
- Saturation Flow Rate
The maximum rate at which vehicles can pass through an intersection during a green signal.
- Cycle Length
The total time taken to complete one full series of traffic signal indications.
- Lost Time
Time during which an intersection is not effectively utilized for vehicle movement.
Reference links
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