In response to the question by Roz on the 'Who is in HOP' thread, I believe there's a pretty simple reason why buses have a tendency to all come in bunches rather than remain evenly spaced.

Say there are a series of buses, evenly spaced, called A, B, C, etc.

As soon as one of them, say B, get a bit delayed for some reason (e.g. by traffic or having to pick up/let off more passengers) there will start to be a larger gap between it and the preceding bus, A.

The larger the gap, the more passengers will tend to be waiting at the next stop and the more time it will take to let them on.

At the same time, because bus B is making slower progress, the gap between it and bus C gets shorter, and resultingly bus C has fewer passengers to pick up, so goes even faster.

Pretty soon bus C catches up with Bus B.

I reckon it's to avoid this happening that buses sometimes seem to stop and wait for a few minutes for no apparent reason half way around their route.

The other day I was on a 176 and I overheard a conversation between the driver and some sort of supervisor, in which the supervisor was telling the driver off for going too fast and arriving at that stop two minutes (seemed a tad on the pedantic side, if you ask me) too early, meaning he'd have to wait two minutes before setting off.