Traffic - Dynamic Systems

(By Forrest Sondahl, 2005)

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Created with NetLogo * View/download model file: Traffic - Dynamic Systems.nlogo

WHAT IS IT? ----------- This is a simple demonstration of the System Dynamics tool. It is a modification of the Traffic Basic sample model. Please refer to "Traffic Basic" for general information about how the model works. We are using System Dynamics to model the speed of a single (average) car as it goes through traffic jams. HOW TO USE IT ------------- The controls on the left side of the screen are the same as in Traffic Basic. The controls on the right side change variables used by the System Dynamics part of the model. The buttons in the middle are used for running both the original model and the system dynamics model simultaneously. There are two additions to the left side of the screen, not present in the original Traffic Basic model. The first is the "cycle" monitor, which tells you the length (in ticks) of the last cycle the red car went through. This is the time it takes the red car to go from X mph through some other speeds and then back around to X mph again. The second addition is the "cycle-testing-speed" slider, which controls what X is, in the previous sentence. Note that you should always choose your cycle-testing-speed to be somewhere between the minimum and maximum speed for the red car, for the particular simulation you're running. Otherwise, it will not report back the proper cycle length. The sliders on the right should be pretty much self-explanatory. Just run the "setup-predict" and "go-predict" a couple of times, to see what effect they have on the resulting plot. THINGS TO NOTICE ---------------- The behavior of the original agent-based model is somewhat sporadic to start with. However, that activity tends to quickly resolve itself into either traffic jamming patterns, or free flowing traffic. It is the traffic jamming patterns that the system dynamics tools is attempting to model. The System Dynamics model is very simple. Does it do a good job of modeling? Are there stable traffic patterns that occur that cannot be modeled with it? THINGS TO TRY -------------- Try to find the setting for the sliders on the right such that the purple line follows the same path as the red line. Is this possible? What if the red car is in a traffic jam where it has to sit still for a while before accelerating? Instead, what if you try to make it so that the purple line peaks at the same place as the red line every time? Is this always possible? If the plots have the same period, and peak and fall at the same time, does this mean that the System Dynamics model is good enough? If you used the system dynamics model to predict the distance that the red car had traveled, would it give the correct result? EXTENDING THE MODEL ------------ Quite frankly, the predictive power of the system dynamics model seems weak. Two extensions quickly suggest themselves: First, a "time-sitting-still" variable could be introduced, which would cause the "stock" to not increase for N timesteps each cycle. Second, devise a way to model the "double-wave form", for when there are multiple traffic jam waves going through the line of cars at the same time. In the plot, this looks like a small rise and drop, followed by a bigger one, then a smaller one, then a bigger, etc... NETLOGO FEATURES ----------------- Dynamic-Systems Tool RELATED MODELS --------------- "Traffic Basic" CREDITS AND REFERENCES ----------------------- This model was developed at the MIT Media Lab. See Resnick, M. (1994) "Turtles, Termites and Traffic Jams: Explorations in Massively Parallel Microworlds." Cambridge, Ma: MIT Press. Adapted to StarLogoT, 1997, as part of the Connected Mathematics Project. Adapted to NetLogo, 2000, as part of the Participatory Simulations Project. To refer to this model in academic publications, please use: Wilensky, U. (1997). NetLogo Traffic Basic model. http://ccl.northwestern.edu/netlogo/models/TrafficBasic. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL. In other publications, please use: Copyright 1997 by Uri Wilensky. All rights reserved. See http://ccl.northwestern.edu/netlogo/models/TrafficBasic for terms of use.

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