Q: My bus was late, then two came at once. Why?
If you've ever been frustrated by this phenomenon, you are not alone. It's called bus bunching and, unfortunately, it plagues bus systems around the world.
So why does it happen? Given enough time two closely-spaced buses (many routes at peak commute times) will get closer and closer together. When the first bus gets delayed, either by traffic or a large group of passengers, more people get in line at stops ahead. With more people getting on than normal, the bus slows down even further. At the same time, since the first bus is picking up more passengers, the bus following it picks up less, meaning it is speeding up. Eventually, the two buses will meet.
It can be a confusing concept. Thankfully, some folks at WBEZ in Chicago put together a great video to explain it: