Karl used this graph in the context of traffic.
- The green line is traffic Speed and it rises (to the right) from zero at the bottom left.
- The red line is traffic Density and it rises (to the left) from zero at the bottom right.
- The black line is traffic Flow and equals Speed x Density.
- Speed = cycle time. The time it takes from the moment a piece of work enters the system to the time it gets to Done.
- Density = work in progress. The amount of work that has entered the system but hasn't yet got to Done.
- Start on the density line (red) at zero (bottom right) and increase the density (move up and to the left). For a while increasing the density increases the flow. Increasing the flow causes the density to reduce. Thus you have a stabilizing feedback loop helping to increase the flow.
- As you continue to increase the density you drop over the top of the flow-curve.
- Now as the density increases the flow decreases. And decreasing the flow causes the density to further increase. Thus you have a different destabilizing feedback loop helping to decrease the flow.