The Gap is greater than 8 seconds, it means every next vehicle will arrive at least after 8 seconds.
We have to find the probability that no vehicle arrives in the gap of 8 seconds.
Expected vehicles per hour = 900/hour
λ = expected vehicles per 8 seconds = (900*8)/(60*60) = 2
λ = 2
P(X=x) = $(e^{-λ} *λ^x)$/x!
P(X=0) = $e^{-2}*2^0$/0! = e^-2 = 1/e^2 will be the answer.