Final answer:
Workers are not at the bottlenecks during batch changes due to the just-in-time delivery system. The lack of large inventory at the main assembly plant means that strikes at sub-assembly plants can interrupt the entire supply chain. An example of this occurred in the mid-1990s when a strike in a factory caused multiple plants to shut down.
Step-by-step explanation:
The reason workers are not present at the bottlenecks during batch changes is because of the just-in-time (JIT) delivery system. In this system, no large inventory of any single part is kept at the main assembly plant. Instead, parts are produced and delivered to meet immediate demand. This means that when batches need to be changed, workers at the sub-assembly plants can go on strike and interrupt the entire supply chain, as the main assembly plant and other parts suppliers rely on receiving orders from the main plant.
An example of this occurred in the mid-1990s when workers in a unionized factory in Dayton, Ohio went on strike. This strike resulted in almost all General Motors assembly plants and numerous sub-assembly plants shutting down, highlighting the vulnerability of the JIT system to disruptions.