Suppose bus 5 picks up slightly more passengers than normal. This delays it slightly. It takes slightly longer to reach the next stop. This means it again picks up slightly more passengers than normal. Bus 5 gets further and further behind bus 6 ahead of it.
Do we understand the bus system? We have a cause and an effect which seem plausible. I've learned that saying something is a cause and something is an effect is fraught with danger. With a deeper understanding, what is cause and what is effect start to blur together. Saying this is a cause and that is an effect is a clumsy way of saying they are part of the same system.
Back to the slightly delayed bus 5. Behind bus 5 is bus 4. Bus 5's delay means bus 4 picks up slightly fewer passengers than normal. Bus 4 gets closer and closer to bus 5 ahead of it.
Now it's less clear bus 5 is causing the problem. Perhaps bus 5 picked up more passengers than usual because bus 6 ahead of it was catching up bus 7 ahead of it. And bus 7 is really the cause. But two ahead of bus 7 is bus 9. Maybe bus 9 in the cause? Or maybe we need to look at the whole system.
You wait ages for a bus and then two or more turn up at the same time! The buses are queueing just like the passengers at the stops! A trip that should take 15 minutes takes 45 minutes.
Queues will form when processes with variability are loaded to high levels of utilization without constraint.
What does the Bus "route" master do? They constrain the system.
- Buses depart at evenly-spaced fixed-duration intervals.
- If a bus is ready to go but it's not its time yet then it waits.
- They limit the number of people getting on the bus.
- They do something if people are not getting off the bus!
