|
Class 02 - 16 January 2007
Website of the day:
[On vacation]
Objectives for today:
- Understand concepts related to
user sub-system
Topics:
- Completion of overview
- User sub-system
Assignment:
In class (Assignment 01):
- Consider
data set
- Plot vehicle trajectories using XY
chart in Excel
- Plot arrival and departure flow
rates vs time (both data sets, XY chart in Excel)
- Plot cumulative arrival and
departure flow rates vs time
- Plot length of queue over time
- What can you learn about traffic
flow characteristics based on the based on the vehicle trajectory
chart?
- What can you learn about the
operation of this one intersection approach using the flow rate,
cumulative flow rate, and queue diagrams?
For next time: Review the NGSIM data
set. Obtain a login ID and password. Explore the Lankershim
data set links and the data that are available at this site.
http://www.ngsim.fhwa.dot.gov/modules.php?op=modload&name=News&file=article&sid=49
Read the data analysis report (the
first link on the above page) and be ready to discuss some of the
characteristics in class at our next meeting.
Class notes:
Questions on today's class:
1. Could you please describe signal grouping with an example?
Response: Note the definition of "signal group": A combination of
indications (e.g., red, yellow, green, green arrow, audibles, etc.)
grouped together for controlling one or more movements. An
example, then, is the grouping of the green, yellow, and red for a
given phase. 2. When would you not want to have at
least a minimum recall? It seems to me that you would always want to
have a min recall or max recall. This results in no phases being
skipped. Skipping phases might result in vehicles that were not detected
(as a result of a detection failure) to never being served.
Response: If a recall is set (even minimum recall), that phase will
be served even if the demand is zero. So, we wouldn't want to set
minimum recall unless we wanted that phase to be served even in the
absence of demand. 3. Can you have more than 2 barriers?
Maybe in a five approach intersection?
Response: Your example is a good one. If you are serving more
than the standard four approaches, an additional barrier could be added
(but not an additional ring). 4. Is the maximum recall setting a
fixed setting in the controller or it can also be set automatically
based on demand at an intersection?
Response: What maximum recall does is to make sure that a phase is
always called (even if the demand is zero) and that the phase will time
to its maximum value. 5. Is the arrival -service pattern
D/D/1 only applied in the case of saturation flow conditions (like peak
hours)?
Response: No. The term D/D/1 means that the arrival pattern is
deterministic, the service pattern is deterministic and that there is
one service channel (or lane). The actually arrival and service
rates can very, but they are deterministic. 6. Just to clarify, in a
phase, all the parts have to occur sequentially but not all the parts
have to occur, correct?
Response: For a vehicle phase, for example, you will always have the
three indications, green, yellow, and red. Please clarify your
question. 7.On the vehicle trajectory graph, the
two shockwaves appear to intersect at some point in time. Is there
anything significant about this intersection?
Response: There is nothing special about the intersection.
Usually, the formation shock wave will not intersect the clearance shock
wave, but it is possible. 8. What is the difference between
indication attributes and typical indication?
Response: I'm not sure I understand your question. Please
clarify and I will respond again. 9. The indication is the representation
of a phase?
Response: From our definition: "the phase is a timing unit associated
with the control of one or more indications." 10. In a four approach intersection; a
high saturation flow rate in one approach affects the conflicting
approach?
Response: The saturation flow rate is just the maximum flow rate.
What I think you're getting at is the variation of volume on the
different approaches. If there is a high flow rate on the
conflicting approach, it will take away green time from the other
approach. Setting signal timing is an ongoing tradeoff between
providing good quality of service to all intersection approaches. |
