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Class 07 - 8 February 2007
Website of the day:
Utah Traffic Lab
Objectives for today:
- Understand relationship between
maximum desirable headway and unoccupancy time
Topics:
- Review and discuss Assignment 4
- Discuss relationship between
headway and unoccupancy time
- Assignment 5
Class notes:
Written notes
Slides
Assignment 5
1. Use the attached spreadsheet to investigate the following
questions.
- Question 1. Based on your saturation headway
data (from all five intervals) from
Assignment 4, compute the equivalent values for unoccupancy time.
Assume a detection zone length of 22 feet, average vehicle length of 20
feet, and average vehicle speed of 35 mi/hr. The distribution
should be at 0.25 sec intervals and you should include in your chart
side by side comparisons of the distribution for the headway data
and the unoccupancy time data. What is the horizontal shift
that you observe between these two distributions? [Hint: check the
difference in the means between your two data sets].
- Question 2. Using this same
spreadsheet tool, prepare an XY chart of unoccupancy time vs
detection zone length for a passenger car (Lv=20') and bus (Lv=40'),
for a range of detection zone lengths from 0 to 100 feet.
Assume h = 1.9 seconds and v = 35 mi/hr. What conclusions can
you draw from this plot regarding how vehicle length and detection
zone length affect unoccupancy time? And, what is the
significance of this finding for signal timing.
- The results of your work (in an
Excel spreadsheet) should be emailed to me by noon on Tuesday,
February 13th. Be ready to discuss the results of your work in
class at that time.
2. Read the following paper: "A
Foundation for Jointly Determining Passage Time and Detection Zone
Length Using Stop Bar Presence Detection". Be ready to discuss
following questions in preparation for class next Tuesday:
- Question 1. What is the major
purpose of the paper?
- Question 2. According to the
paper, when is the appropriate time to terminate the green interval?
- Question 3. Why do we need to
consider field or simulation data when looking at the true effects
of headway distributions and/or unoccupancy time distributions?
- Question 4. Briefly summarize the
results of the case study described in section 4 of the paper.
What is the justification for the selection of the recommended
passage time?
Questions on today's class:
1. Can someone really just change the
passage time for an approach as a result of a phone call?
Response: It does happen! The "phone call" indicator is
something that traffic engineers really talk about at meetings!
More often than not, the problem is with a side street that deals with a
long maximum green on the major street.
2. Detectors, I assume, are very costly.
Why not just always install small detectors and have larger passage
times?
Response: Good question. There is a tradeoff in the cost vs.
the performance that the longer detection zone provides. Having
the detector "take the place" of the longer passage time is something
that is being considered these days. But in addition to the cost
trade-off, there is also the reliability problem. Longer loops
tend to have more problems than shorter loops. But the point here
is that we have to consider the timing parameter decision at the same
time that we are considering the detection zone design.
3. Just to clarify, the headways used for
Assignment #3 (questions 5 on) were headways of the vehicles as they
pass a single point, not an average headway for the time a particular
vehicle is in the system.
Response: Correct. They are the individual headway
measurements, not an average.
4. I understand that the unoccupancy time
is one of the parameters used to set up the passage time. Which value of
unoccupancy time should be used as reference, the larger one?
Response: What we are saying is that you should start with the
maximum headway that you are willing to tolerate before you make the
phase termination decision. Once you've established this value,
you can then relate this headway to an unoccupancy time, based on an
average vehicle speed and length. I'm not sure what you mean by
the "larger one". 5. I got
3.8 sec for the saturation headway, which I think is too high. Should I
consider not using the 4th and 5th vehicle in computing the saturation
headway as those are the ones that are increasing the saturation
headway.
Response: The Highway Capacity Manual suggests that the headways
beginning with the 4th vehicle be used in computing the saturation
headway. So, I don't suggest eliminating this measurement.
However, it does seem high. Have you checked your other values?
See me if your estimate remains at this level.
6. What is the significance of detector
lengths in multiples of 22' feet? Does it have something to do with the
average car length of 20 feet?
Response: No. It has to do with the standard loop length of 6
feet by 6 feet. Many agencies use two loops at the stop bar spaced
9 feet apart. So, 6 + 9 + 6 is 21 feet. I've used 22 feet in
many of my examples so this is close to the standard used in practice.
I'll get my notes updated at some point!
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