Is it really true that you wait ages for a bus and then three turn up at once?

It's a popular cliché to say that you wait ages for a bus, and then three turn up at once. But is there any truth to this? Or is it a false impression formed because we notice coincidences more than other events?
If true, are there laws governing this behaviour, and are there any natural phenomena that obeythe same principles?

this question was asked by Moderator of the New Scientist last word web page