# 10. The structure of material stocks and flows and nodes of intersection The **plumbing structure, the stocks and flows and their physical arrangement**, can have an enormous effect on how the system operates. When the Hungarian road system was laid out so all traffic from one side of the nation to the other has to pass through central Budapest, that determined a lot about air pollution and commuting delays that are not easily fixed by pollution control devices, traffic lights, or speed limits. **The only way to fix a system that is laid out wrong is to rebuild it, if you can.**
Often you can't, because physical building is usually the **slowest and most expensive kind of change** to make in a system. Some stock-and-flow structures are just plain unchangeable. The baby-boom swell in the U.S. population first caused pressure on the elementary school system, then high schools, then colleges, then jobs and housing, and now we're looking forward to supporting its retirement. There is not much we can do about it, because five-year-olds become six-year-olds, and sixty-four-year-olds become sixty-five-year-olds predictably and unstoppably. The same can be said for the lifetime of destructive CFC molecules in the ozone layer, for the rate at which contaminants get washed out of aquifers, for the fact that an inefficient car fleet takes 10 to 20 years to turn over.
Physical structure is crucial in a system, but rarely a leverage point, because changing it is rarely simple. **The leverage point is in proper design in the first place.** After the structure is built, the leverage is in **understanding its limitations and bottlenecks and refraining from fluctuations or expansions that strain its capacity**.