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Class 08 - 13 February 2007
Website of the day:
Ada County Highway District - Traffic Management Center
Objectives for today:
- To enhance understanding of the
relationship between detection zone length and traffic flow
parameters, including stochasticity of parameters
Topics:
- Discuss results of Assignment 5
- Preparation for Exam 1
- Permitted left turn model
Notes for Exam 1
Exam 1 will be closed book and closed notes. The best way to
prepare is to review the five assignments that you have completed during
class thus far, and to review the questions and responses that are
posted with each day's class web page.
Consider the following questions as you
prepare for the exam:
- What can you learn about traffic
flow principles and the operation of a traffic signal system from
the plots of arrival and service rates vs. time, cumulative arrivals
and departures vs. time, and queue length vs. time?
- What can you learn about traffic
flow principles and the operation of a traffic signal system from
vehicle trajectory (time-space) plots?
- How does a vehicle trajectory plot
show information on stop bar location and on the building and
clearing of a queue?
- How does the Lankershim data set
need to be filtered so that headway between two vehicles are both
realistic and usable?
- What does the plot of headway vs
LocalY for all vehicles in a queue show about traffic flow
principles? How do you explain the variation in headway vs
LocalY?
- How would you relate the plot of
headway vs LocalY with the plot of headway vs position in queue?
- How do you know when vehicles
change lanes in the trajectory plot?
- How do you determine whether a
vehicle is part of a queue, either standing or departing?
- What can you learn about the
relationship between detection zone length and unoccupancy time from
the Lankershim data set?
Class notes:
Written notes
Slides
Questions on today's class:
1. When designing an actuated permitted
LT, can we use the maximum flow rate to determine if this LT has to be
changed to protected, if saturation reaches the maximum accepted value
for storage queue length?
Response: The maximum or capacity flow rate that you compute using
the procedure that we discussed today can be used to estimate what the
queue conditions will be and can be used to determine if a protected
phase is needed.
2. When the opposing traffic has 2 lanes the
gap offered for LT vehicles is less accepted (critical gap bigger than
just one lane). How does it complicate the analysis of gap acceptance?
Response: If there are two opposing lanes the model is more
complicated. But even here we can obtain a good approximation by
considering the total flow together in both lanes and identifying the
gaps between vehicles in both lanes. The complicating factor is
that sometimes vehicles will be side by side and it is overstating the
effect to consider each vehicle separately.
3. How about the lag? Does it has influence in the max flow rate for left
turn movements?
Response: The lag is the time from when a left turn vehicle arrives
and the first opposing vehicle arrives. In most cases, the lag can
be used just as we would a gap.
4. Does the critical gap
consider the driver behavior?
Response: The critical gap is determined solely based on driver
behavior. It can only be determined by measuring gaps in the field
and identifying which ones are accepted and which are rejected.
5. At what point in the left turn is the follow up time measured (beginning of turning movement, end of turn, etc..)?
Response: The follow up time is measured from the departure of one
vehicle (as it passes the stop line) to the departure of the following
vehicle (again as it passes the stop line).
6. Since the critical
gap will vary by driver, is there a theoretical value suggested for use
or should you attempt to determine an average for your particular
situation?
Response: The critical gap must be determined from field data.
The Highway Capacity Manual provides default values to use (in fact
based on the research that the University of Idaho did: see
NCHRP 3-46). But the recommendation is always, if possible,
collect data for the site that you are studying and use these data.
7. In our model we assumed that no
vehicles get through during the first part of green. Will it affect our
model if some vehicles do get through?
Response: Yes. In fact, if a left turn vehicle turns in front
of an opposing queue, or if a left turn vehicle makes its turn at the
end of green (during yellow), the model must be corrected.
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