I said that the dominant constraint is road capacity. Which means that buses must be large enough to carry the required number of passengers on the streets that

I said that the dominant constraint is road capacity. Which means that buses must be large enough to carry the required number of passengers on the streets that exist. When you have buses running on ordinary streets with dedicated bus lanes (but no bus rapid transit), peak capacity is around 120 vehicles/hour/lane. Even mid-sized cities can have bottlenecks where that limit is reached due to geographic constraints.