Simples!

I was a bit surprised to come across a serious article the other day wondering about why buses seem to come in threes.  This is something that has been talked about since I was a lad, and surely the answer is obvious?  How fast a bus can complete its route depends on how many passengers it has to pick up and set down.  Every time it has to stop for these reasons, it loses time.  If several buses are plying the same route, the first one in sequence will pick up the majority of the passengers, which will slow it down, while the following ones will pick up fewer, and therefore catch up with the first and indeed with each other.  It's inevitable, and the further down the route you are, the more likely you are to see a bunch of buses coming along at once.  Seems clear enough to me!