If the probability of observing a car (read: at least one car) in 20 minutes on a highway is 609/625, what is the probability of observing a car (read: at least one) in 5 minutes (assuming constant default probability)?

(In reply to

re: Auto-suggestion by Charlie)

Charlie,

Why must we fit a Poisson distribution? (Not that we can't use
it, but why MUST we use it, as opposed to a different distribution?)