Quote:
Originally Posted by Meraki
When buses are running in sub-10 minute frequency all it takes is a traffic light or two to cause bunching. I see it as an opportunity to wait for the 2nd bus, which will usually have a lot more room available.
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Bus schedules by their very nature are unstable. If there are any disturbances, the buses will tend to get further and further from their scheduled time - it's like trying to balance a ball on the top of your head - the further away from the centre it gets the harder it is to keep it where it's supposed to be.
If a bus is delayed for some reason (congestion at a traffic light, long waits for many passengers or disabled passengers that requires the ramp to be lowered) then it will be slightly behind schedule. If it's slightly behind schedule then chances are there will be a few more people waiting at all the subsequent stops, which then requires to bus to dwell at stops longer, which puts it further behind schedule, etc. etc.
In the meantime, the following bus will tend to see fewer people at each stop because there's less time between it and the bus ahead. So it will spend less time picking up and dropping off people, which means it will tend to get ahead of schedule.
When this plays out you end up with the two buses bunched together.
This is why you can't run buses with frequencies shorter than a few minutes - the higher the frequency the more pronounced this issue becomes because any given delay is a much bigger proportion of the interval between buses, so they are much more likely to bunch up. And that's why it's not really practicable to add more buses to the Broadway B-Line to attempt to increase capacity.