I had a discussion the other day with the customer about our velocity in the sprint and the amount of work I (and our team) allowed in the sprint. He argued that we could add more, just to make it more a challenge. In fact, I think he thought we were a bit lazy or at least cowardly. So I replied with this traffic jam metaphor.
A sprint is like a busy road!
Look at a real traffic jam. If a car will break down and the road is already very busy, this will be probably cause a traffic jam. There is no room for the other cars to find alternative routes or place. Also many trucks will have effect in the efficiency of all the traffic. You can play and see for yourself with the simulator.
Looking at stories, the same applies. If you have too many stories in your sprint and a story will cause problems, your team has no room to find alternatives and the sprint will stall. You will see it directly in your burn down. Same for too much difference in size or if there is a new story added during a sprint. This metaphor works actually quite good :)
- Most important one: Keep the amount of stories (or work) in total reasonable, so there is room to move/find alternatives if needed. You want to be accountable and predictable so you need a realistic pace. You need room to move if something happens. And trust me...something will happen.
- Prevent much difference between the speed of cars/stories. Try to keep the stories small. You don't want Roadtrains in your sprint.
- You need a procedure what happens if a car (story) will break down (crash). It can not block the road so you must get it out of your way somehow.
- Don't allow new cars/stories to your road when you are already driving :)
This also explains why it is a good idea to poker a little bit more in a session than you have room for, just to have some spare stories to switch with others to make a good sprint planning.
These things are not new but perhaps using this metaphor helps you guys to stop creating traffic jams in the sprints and get those cars moving again :)