# MAP and ML in practice on the New Jersey Turnpike

Since I might be teaching detection and estimation next semester, I’ve been thinking a little bit about decision rules during my commute down the New Jersey Turnpike. The following question came to mind:

Suppose you see a car on the Turnpike who is clearly driving dangerously (weaving between cars, going 90+ MPH, tailgating an ambulance, and the like). You have to decide whether the car has New Jersey or New York plates [*]?

This is a hypothesis testing problem. I will assume for simplicity that New York drivers have cars with New York plates and New Jersey drivers have New Jersey plates [**]:
$H_0$: New Jersey driver
$H_1$: New York driver
Let $Y$ be a binary variable indicating whether or not I observe dangerous driving behavior. Based on my entirely subjective experience, I would say the in terms of likelihoods,
$\mathbb{P}(Y = 1 | H_1) > \mathbb{P}(Y = 1 | H_0)$
so the maximum likelihood (ML) rule would suggest that the driver is from New York.

However, if I take into account my (also entirely subjective) priors on the fraction of drivers $P(H_0), P_H(1)$ from New Jersey and New York, respectively, I would have to say
$\mathbb{P}(Y = 1 | H_1) P(H_1) < \mathbb{P}(Y = 1 | H_0) P(H_0)$
so the maximum a-posteriori probability (MAP) rule would suggest that the driver is from New Jersey.

Which is better?

[*] I am assuming North Jersey here, so Pennsylvania plates are negligible.
[**] This may be a questionable modeling assumption given suburban demographics.

## 2 thoughts on “MAP and ML in practice on the New Jersey Turnpike”

1. Are they throwing things at your car and flipping you off? Then they’re are definitely from New Jersey.

2. My instinct is to suppose that that the recklessness of any given driver only slightly varies with whether they’re in their home state, so that the most important factor is the fraction of drivers from New Jersey versus New York.

Be a heck of a thing to finally get a look and see the driver’s got a license plate from Guam somehow.

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