Fundamentals of Transportation/Modeling

< Fundamentals of Transportation

All forecasts are wrong; some forecasts are more wrong than others. - anonymous

Modeling is a means for representing reality in an abstracted way. Your mental models are your world view: your outlook on life, and the world. The world view is your internal model of how the world works; it is employed every time you make a prediction: what do you expect, what is a surprise. The expression “Where you stand depends on where you sit” epitomizes this idea. Your world view is shaped by your experience and your position.

When modeling, the issue of Point of View should be considered. It must be clear who (and what) the results are for. If you are modeling for personal pleasure, it will naturally reflect your own worldview, but if you are working for an employer or client, their point of view must also be considered, if the inputs or results deviate significantly from their worldview, they may adapt their worldview, but more likely will dismiss the model.

Modeling can be conducted both for subjective advocacy and for objective analysis. The same methods may be employed in either, and the ethical modeler will produce the same results in either case, but they may be used differently.


Why Model?

There are a variety of reasons to model. Modeling helps

Particular applications in transportation include:

Developing Models

As an engineer, economist, or planner you may be given a model to use. But that model was not spontaneously generated, it came from other engineers, economists, or planners who undertook a series of steps to translate raw data into a tool that could be used to generate useful information.

The first step, specification, tells us what depends on what (the number of trips leaving a zone may depend on the number of households). The estimation step tells us how strong these relationships mathematically are, (each household generates two trips in the peak hour). Implementation takes those relationships and puts them into a computer program. Calibration looks at the output of the computer program and compares it to other data, if they don't match exactly, adjustments to the models may be made. Validation compares the results to data at another point in time. Finally the model is applied to look at a project (e.g. how much traffic will use Granary Road after Washington Avenue is closed to traffic).

Specification

When building a model system, numerous decisions must be made. These are discussed below:

Types of Models

There are numerous types of models, a short list is below. Each has different applicability, multiple methods may be used in pursuit of the same question, sometimes they are complementary, and sometimes competitive techniques.

Model Trade-offs

Building a model requires trading-off time and resource constraints. One could always be more detailed, more accurate, or more comprehensive if resources were not constrained. However, the following must also be considered.


Organization of Model System

Time

Space

Process

Type

Solution Techniques

When solving the model, the system as a whole must be understood. Several questions arise:

Solution techniques often trade-off accuracy vs. speed. Some solution techniques may only guarantee a local optima, while others (such as brute force techniques) can guarantee a global optimum, but may be much slower.

“Four-Step” Urban Transportation Planning Models

We want to answer a number of related questions (who, what, where, when, why, and how):

If we know the answers to those questions, we want to know what are the answers to those questions a function of?

The reason for this is to understand what will happen under various circumstances:

In short, for urban passenger travel, we are trying to predict the number of trips by:

This is clearly a multidimensional problem.

In practice, the mechanism to do this is to employ a "four-step" urban transportation planning model, each step will be detailed in subsequent modules. These steps are executed in turn, though there may be feedback between steps:


Thought Questions

Additional Problems

Key Terms

Video

References

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