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Assumptions The probability of seeing a state license plate A in another state B depends on the distance between their state capitals. It is scaled by the number of licensed drivers in state A. (This indirectly means that the probability does not depend on how long we are in a state).
Seeing state license plates A, B, etc. are independent from other license plates in a given state D.
Seeing given state license plate A is independent when driving across states B, C,…
We do not adjust for round trips.
The distance between state capitals was found here. The number of licensed drivers per state is here. I estimated the odds of seeing a license plate from state A in state B is captured by this formula: P = exp(-K * (Distance from A to B in miles) / # of licensed drivers) with K = 7000 – 2000*Summer01 – 1000*ExpensiveGas01. Summer01 is 1 if it is summer break and 0 otherwise. ExpensiveGas01 is 1 if it gas is “expensive” and AAA predicts that road trips will be down and 0 otherwise