h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The real reason why buses turn up in threes is because the first two buses are abducted by aliens and tested on for efficiency and technology development, then sent back to earth - this means they are late and are often found in fromt of another.

No really, the real reason is because the first bus is delayed because many people want to get the bus. This slows the bus down, so buses have to spend more time at following stops, because more people have gathered at them. This allows anothe bus to catch up! Then there are fewer people at the previous bus stops, so bus #3 runs early and catches buses #1 and #2.

But they don't always come in threes - in fact if you are in Bolton, they often turn up in fours and even fives! Yet on Manchester's Oxford Road, you can apparently get twelve together! Some mad aliens must have a fetish for buses!