[todd8]

There is also the effect that buses arrive imprecisely and consequently may be best modeled as a Poisson distribution of arrival times. If this is the case and buses arrive on average every 15 minutes, then passengers arriving at random will wait not 7.5 minutes but 15 on average.

Intuitively, think of throwing a dart at the time line. We are more likely to hit the line in the bigger gaps between buses’s arrivals than between buses that arrive close to each other.

See https://en.m.wikipedia.org/wiki/Poisson_distribution