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Class 01 - 11 January 2007
Website of the day:
US Department of Transportation
Traffic signal history
http://www.ideafinder.com/history/inventions/story010.htm
http://mutcd.fhwa.dot.gov/kno-history.htm
Objectives for today:
- Understand traffic signal control
system and its four subsystems
- Understand course objectives and
requirements
Topics:
- Course objectives and requirements
- Traffic signal control system and
subsystems
Assignment: Complete the
following reading assignment before class 2 and be prepared to discuss
what you've read.
Class notes:
Questions on today's class:
1. Where does the information for the time-space diagrams come from?
Is it observed data? How do you make a diagram like that for a
proposed traffic signal control system that is not yet in place, if
you are just trying to model the operations before the installation?
Response: The time-space diagrams that I presented in the first
class are examples. What is most often used for planning for a
new signal system are either flow rate data or sometimes headway
distribution data. That is one of the challenges in planning
for a new system.
2. In a city full of traffic
intersections with a large population and narrow roads, what is the
first action one has to take in providing a traffic system?
Response: Usually you are not planning from the very beginning.
As a city develops, infrastructure, including transportation
infrastructure, is added periodically. What normally happens, is
that a city or a metropolitan area identifies problems that exist, and
then develop plans to help manage these problems. You will notice
that I didn't say solve the problems - these days we expect to manage
problems not solve them.
3.Since the four subsystems work
together, how can one determine which of the units does not function
properly?
Response: The purpose of looking at the four subsystems together is
to learn the importance of their interdependence. There are
several ways to look at the question "one of the units does not function
properly". For example, we typically use measures of effectiveness
such as average delay as a measure to determine if the system is working
effectively or not. However, we also need to know if a physical
component, such as a loop detector or signal head, is not working
properly. These problems are often identified by observing the
system in the field.
4. I want to have more understanding of
different detector types.
Response: We will discuss detector types in a later section of the
class.
5: Are there any plans for coordinating
all of the signals in Moscow, or is it even necessary?
Response: The downtown signals are coordinated (though this doesn't
mean that good progression is provided in all directions). The
signals on State Highway 8 between downtown and Warbonnet Drive (the
location of WalMart) are coordinated in the afternoon.
6: On some timing plans I've seen, what
is the difference between a max1 and a max2 time?
Response: The Max1 parameter is used more often as the default.
Max2 is used for an alternative timing plan.
7. It was described that the passage
timer becomes active when the phase starts. If there is no vehicles and
the minimum green is 10 sec, the passage time is 2.5 sec. will the phase
terminate at 10 sec. when both expire?
Response: Yes.
8. Is a signal group the display of the
green + yellow + red intervals? If so, is the signal group the same as a
cycle?
Response: The definitions is: "A combination of indications (e.g.,
red, yellow, green, green arrow, audibles, etc.) grouped together for
controlling one or more movements." So you are correct.
However, the cycle is a time interval, not the combination of the
indications. The cycle is the time duration or interval that it
takes to serve all movements.
9.It was stated that the absence of
calls results in the skip of phase, unless a minimum green or minimum
recall is set on the controller, for this phase?
Response: This is correct. If there is no call for a phase, it
is skipped, unless a recall setting has been placed in the controller.
The recall setting (specifically maximum recall) is most often used when
a loop detector is broken.
10. In which cases are video, radar,
acoustic detections mostly used?
Response: The standard detection system is the loop detector.
More recently, new technologies, such as those that you mention, have
been tried. The most commonly used of these new technologies is
video detection. We will talk about these later in the course.
11. From the diagrams explaining how
one subsystem responds to another, we can read the passage time and the
minimum green time. How about the maximum green time? I suppose more
observing data are needed to answer this question.
Response: The diagrams only show two of the controller processes, the
minimum green timer and the passage timer. There are a number of
other timing processes, including the maximum green timer, that will
appear on this diagram. We will add more as we progress in the
semester.
12. I think that there is a mistake in
a figure 7 of the reading, the queue accumulation polygon. According to
the data before this figure, the first vehicle arrives at the stop line
10 seconds after the beginning of the observation period. Should not a
square denoting the first vehicle in a queue be placed just above t=10
sec (x axis)? The queue length should also decrease after t=40 sec but
not t=30 sec as it is shown in this figure.
Response: [to be added]
13. Imagine that there is no
conflicting call, the green phase in the approach that is in service
will be ON until the passage time expires after the minimum green has
been expired. And hypothetically speaking if the passage time does not
expire because the vehicles ask for service before the passage time can
expire. In this situation if there is a pedestrian trying to cross the
approach in service it will be impossible, or a call from the pedestrian
is consider as a conflicting call?
Response: The pedestrian call acts as a conflicting call; it will
force the termination of the vehicle phase and serve the pedestrian
phase (if it is in a conflicting direction).
14. What is the amount of time that it
takes for one complete cycle in the process of
USER-DETECTOR-CONTROLLER-DISPLAY?
Response: This depends on the timing parameters that have been set
for each phase. The cycle is the time that it takes to serve each
movement that desires service.
15. How many different types of
detectors are there? Is the logic behind them similar or very different?
Response: There are a number of types including loop, video, and
audio. We will discuss their characteristics later in the class.
16. Why would you differentiate a
"user" from a "vehicle"?
Response: The user class includes "a vehicle" as well as other user
types (for example, pedestrians).
17. And are we calling the control
subsystems "classes" for any particular reason?
Response: The term class is used to denote the fact that there a
types or categories for one of the subsystem. We don't use the
term "class" for the control subsystem. 18. Figure 5. Saturation flow rate
shows saturation flow formed by nine vehicles. Would it be correct to
say that service rate for vehicles 8 and 9 is equal to arrival rate (not
saturation flow rate) since these vehicles are only slightly delayed by
the dissipating queue?
Response: If the vehicles are even slightly delayed, then the arrival
flow rate is higher than the departure flow rate. |
