Working in teams at the computer, (teams of two work best for most problems), the students discuss and create the factors (or variables) that can cause traffic jams. They are using experience and intuition, drawing upon prior knowledge.
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Students create factors on a map. They title the factor and describe it. A good description includes information on how to measure the factor or what evidence would indicate that an event has occurred. For example, the factor "number of cars" would have a description defining number of cars passing per minute.
Initially there aren't too many rules on what constitutes a factor, since there might be some interesting and unexpected thinking going on. This is where the teacher can really benefit from this "visible thinking". The teacher talks with the teams and asks probing questions such as:
Students' answers to these questions often belong in the descriptions of the factors.